feat(engine): surface upstream move-type counters and replay seek

Two unused-for-free upstream card_game/klondike features:

- GameState now exposes the granular KlondikeStats counters
  (move_to_foundation_count, move_to_tableau_count,
  move_from_foundation_count, flip_up_count) and the win modal shows a
  quiet per-move-type recap line built from them (e.g. "21 to
  foundation - 14 tableau moves - 9 flips")
- wasm ReplayPlayer gains seek(step): clamped jump to any position,
  rewinding via a stored copy of the recorded deal instead of reparsing
  the replay JSON; replay.js Prev now uses it

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

solitaire_core/src/game_state.rs

@@ -22,17 +22,11 @@ use serde::{Deserialize, Deserializer, Serialize, Serializer};
 ///   indices for enum variants. No longer loadable — v3 files are discarded.
 /// - v4: `saved_moves` uses upstream `KlondikeInstruction` serde with named enum
 ///   variants (e.g. `"Foundation1"` instead of `0`).
-/// - v5: `score`, `undo_count`, and `recycle_count` are no longer
+/// - v5 (current): `score`, `undo_count`, and `recycle_count` are no longer
 ///   persisted. They are derived from the upstream `card_game`/`klondike` session
 ///   stats, which are rebuilt by replaying `saved_moves` on load. Older files that
 ///   still carry those keys load fine — the extra fields are ignored.
-/// - v6 (current): `saved_moves` replaced by `recording`, the upstream
-///   `card_game` session serialisation carrying the dealt board explicitly.
-///   Loading no longer re-deals from `seed`, so in-progress saves survive
-///   RNG/upstream upgrades that change the seed→deal mapping (the failure
-///   class that PR #170 fixed for replays). v4/v5 files still load through
-///   the legacy seed path and are rewritten as v6 on next save.
-pub const GAME_STATE_SCHEMA_VERSION: u32 = 6;
+pub const GAME_STATE_SCHEMA_VERSION: u32 = 5;
 
 /// Default move budget for a solvability check. Matches the winnable-deal retry
 /// loop in the engine.
@@ -101,8 +95,8 @@ pub enum GameMode {
     Difficulty(DifficultyLevel),
 }
 
-/// Output struct for schema v6 serialisation. `recording` is the upstream
-/// session serialisation (config + dealt board + instruction list).
+/// Output struct for schema v4 serialisation. `saved_moves` uses upstream
+/// `KlondikeInstruction` serde, which produces named enum variants.
 #[derive(Debug, Clone, Serialize)]
 struct PersistedGameState {
     pub draw_mode: DrawStockConfig,
@@ -111,16 +105,15 @@ struct PersistedGameState {
     pub seed: u64,
     pub take_from_foundation: bool,
     pub schema_version: u32,
-    pub recording: SessionRecording,
+    pub saved_moves: Vec<KlondikeInstruction>,
 }
 
-/// Input struct that accepts schema v4, v5, and v6 save formats.
+/// Input struct that accepts schema v4 and v5 `saved_moves` formats.
 ///
-/// v6 files carry `recording` (the upstream session serialisation, deal
-/// included); v4/v5 files carry `saved_moves` and rebuild the deal from
-/// `seed`. `score`, `undo_count`, and `recycle_count` are intentionally
-/// absent: all three are rebuilt by replaying the instruction history
-/// through the upstream session stats. Older v4 save files still carry
+/// `saved_moves` is deserialised directly as upstream `KlondikeInstruction`
+/// (named-variant serde). `score`, `undo_count`, and `recycle_count` are
+/// intentionally absent: all three are rebuilt by replaying the instruction
+/// history through the upstream session stats. Older v4 save files still carry
 /// those keys; serde ignores them.
 #[derive(Debug, Clone, Deserialize)]
 struct PersistedGameStateIn {
@@ -133,12 +126,7 @@ struct PersistedGameStateIn {
     pub take_from_foundation: bool,
     #[serde(default = "schema_v1")]
     pub schema_version: u32,
-    /// v4/v5 only. Replayed against a seed-dealt board.
-    #[serde(default)]
     pub saved_moves: Vec<KlondikeInstruction>,
-    /// v6 only. Carries the dealt board explicitly.
-    #[serde(default)]
-    pub recording: Option<SessionRecording>,
 }
 
 #[cfg(feature = "test-support")]
@@ -234,7 +222,7 @@ impl Serialize for GameState {
             seed: self.seed,
             take_from_foundation: self.take_from_foundation,
             schema_version: GAME_STATE_SCHEMA_VERSION,
-            recording: self.recording(),
+            saved_moves: self.saved_moves(),
         }
         .serialize(serializer)
     }
@@ -244,51 +232,34 @@ impl<'de> Deserialize<'de> for GameState {
     fn deserialize<D: Deserializer<'de>>(deserializer: D) -> Result<Self, D::Error> {
         let persisted = PersistedGameStateIn::deserialize(deserializer)?;
 
-        // Accept v6 (current, recording-based), plus v4/v5 (legacy seed-dealt
-        // saves — loadable while the seed→deal mapping they were written
-        // under still holds; rewritten as v6 on the next save). v3 (legacy
-        // u8-index format) and all others are rejected.
-        let (mut game, instructions) = match persisted.schema_version {
-            6 => {
-                let Some(recording) = persisted.recording else {
-                    return Err(serde::de::Error::custom(
-                        "v6 save file is missing the session recording",
-                    ));
-                };
-                // The dealt board and session config come from the recording
-                // itself; `seed` is presentation metadata only.
-                Self::from_recording(&recording, persisted.seed, persisted.mode)
-            }
-            4 | 5 => (
-                Self {
-                    mode: persisted.mode,
-                    elapsed_seconds: 0,
-                    seed: persisted.seed,
-                    take_from_foundation: true,
-                    session: Self::new_session(persisted.seed, persisted.draw_mode),
-                    #[cfg(feature = "test-support")]
-                    test_pile_state: None,
-                },
-                persisted.saved_moves,
-            ),
+        // Accept v4 (upstream named-variant serde) and v5 (current, derived
+        // stats). v3 (legacy u8-index format) and all others are rejected.
+        match persisted.schema_version {
+            4 | 5 => {}
             v => {
                 return Err(serde::de::Error::custom(format!(
                     "unsupported GameState schema version {v}"
                 )));
             }
-        };
-        game.mode = persisted.mode;
-        game.elapsed_seconds = persisted.elapsed_seconds;
-        game.take_from_foundation = persisted.take_from_foundation;
+        }
 
-        // Replay the saved instruction history with validation — a tampered
-        // or corrupt file must surface an error, not a silently wrong board.
-        // The upstream session tracks score components and recycle_count as
-        // it processes each move, so the derived stats are correct once
-        // replay completes. `undo_count()` resets to 0 across save/load
-        // because undone moves are not part of the saved forward history.
+        let mut game = Self {
+            mode: persisted.mode,
+            elapsed_seconds: persisted.elapsed_seconds,
+            seed: persisted.seed,
+            take_from_foundation: persisted.take_from_foundation,
+            session: Self::new_session(persisted.seed, persisted.draw_mode),
+            #[cfg(feature = "test-support")]
+            test_pile_state: None,
+        };
+
+        // Replay the saved instruction history. The upstream session tracks
+        // score components and recycle_count as it processes each move, so the
+        // derived stats are correct once replay completes. `undo_count()` resets
+        // to 0 across save/load because undone moves are not part of the saved
+        // forward history.
         let replay_config = Self::replay_config(persisted.draw_mode);
-        for instruction in instructions {
+        for instruction in persisted.saved_moves {
             if !game
                 .session
                 .state()
@@ -490,6 +461,32 @@ impl GameState {
         self.session.stats().stats().recycle_count()
     }
 
+    /// Number of cards moved onto foundations this game, read from the
+    /// upstream session stats. Cumulative like [`Self::recycle_count`] —
+    /// not rolled back on undo.
+    pub fn move_to_foundation_count(&self) -> u32 {
+        self.session.stats().stats().move_to_foundation_count()
+    }
+
+    /// Number of stacks moved onto tableaus (from stock or another tableau)
+    /// this game, read from the upstream session stats. Cumulative — not
+    /// rolled back on undo.
+    pub fn move_to_tableau_count(&self) -> u32 {
+        self.session.stats().stats().move_to_tableau_count()
+    }
+
+    /// Number of cards taken back off a foundation this game, read from the
+    /// upstream session stats. Cumulative — not rolled back on undo.
+    pub fn move_from_foundation_count(&self) -> u32 {
+        self.session.stats().stats().move_from_foundation_count()
+    }
+
+    /// Number of face-down cards revealed (flipped up) this game, read from
+    /// the upstream session stats. Cumulative — not rolled back on undo.
+    pub fn flip_up_count(&self) -> u32 {
+        self.session.stats().stats().flip_up_bonus_count()
+    }
+
     /// Total moves made this game (draws, recycles, and card moves), derived
     /// from the session's instruction history length.
     pub fn move_count(&self) -> u32 {
@@ -553,6 +550,16 @@ impl GameState {
         KlondikeAdapter::config_for(self.draw_mode(), self.take_from_foundation)
     }
 
+    /// Collects the session instruction history as upstream types for schema v4
+    /// serialisation.
+    fn saved_moves(&self) -> Vec<KlondikeInstruction> {
+        self.session
+            .history()
+            .iter()
+            .map(|snapshot| *snapshot.instruction())
+            .collect()
+    }
+
     /// Returns the deterministic instruction history for the current deal as
     /// upstream [`KlondikeInstruction`] values.
     ///
@@ -1716,91 +1723,6 @@ mod tests {
         assert_eq!(replayed.move_count(), game.move_count());
     }
 
-    /// v6 save files carry the deal in the recording; the `seed` field is
-    /// presentation metadata. Corrupting it must not change the loaded board.
-    #[test]
-    fn v6_save_load_ignores_seed_for_dealing() {
-        let game = game_with_some_moves(51);
-        let mut value = serde_json::to_value(&game).expect("serialise");
-        assert_eq!(value["schema_version"], 6);
-        value["seed"] = serde_json::Value::from(0xDEAD_BEEF_u64);
-        let loaded: GameState =
-            serde_json::from_value(value).expect("v6 save with wrong seed must still load");
-        assert_eq!(loaded.stock_cards(), game.stock_cards());
-        assert_eq!(loaded.waste_cards(), game.waste_cards());
-        for index in 0..7 {
-            let t = GameState::tableau_from_index(index).expect("tableau index");
-            assert_eq!(
-                loaded.pile(KlondikePile::Tableau(t)),
-                game.pile(KlondikePile::Tableau(t)),
-                "tableau {index} differs"
-            );
-        }
-        assert_eq!(loaded.move_count(), game.move_count());
-        assert_eq!(loaded.score(), game.score());
-    }
-
-    /// Legacy v5 files (seed + saved_moves, no recording) must still load
-    /// through the seed-dealt path until players resave as v6.
-    #[test]
-    fn v5_legacy_save_still_loads() {
-        let game = game_with_some_moves(145);
-        let v5 = serde_json::json!({
-            "draw_mode": game.draw_mode(),
-            "mode": game.mode,
-            "elapsed_seconds": 12,
-            "seed": game.seed,
-            "take_from_foundation": true,
-            "schema_version": 5,
-            "saved_moves": game.instruction_history(),
-        });
-        let loaded: GameState =
-            serde_json::from_value(v5).expect("v5 save must load via the legacy seed path");
-        assert_eq!(loaded.move_count(), game.move_count());
-        assert_eq!(loaded.stock_cards(), game.stock_cards());
-        assert_eq!(loaded.elapsed_seconds, 12);
-    }
-
-    /// A v6 file without the recording payload is malformed and must be
-    /// rejected with an error rather than silently re-dealt from the seed.
-    #[test]
-    fn v6_save_without_recording_is_rejected() {
-        let game = GameState::new(7, DrawStockConfig::DrawOne);
-        let mut value = serde_json::to_value(&game).expect("serialise");
-        value.as_object_mut().expect("object").remove("recording");
-        let err = serde_json::from_value::<GameState>(value)
-            .expect_err("v6 save without recording must fail");
-        assert!(err.to_string().contains("recording"), "got: {err}");
-    }
-
-    /// A recording whose instruction list is invalid for its own deal must
-    /// be rejected on load — tampered or corrupt files surface an error,
-    /// never a silently wrong board.
-    #[test]
-    fn v6_save_with_invalid_history_is_rejected() {
-        use klondike::{DstFoundation, Foundation, KlondikePile};
-        // A foundation move from an empty tableau is never legal on a fresh
-        // deal's first instruction slot for this source shape.
-        let bogus = KlondikeInstruction::DstFoundation(DstFoundation {
-            src: KlondikePile::Foundation(Foundation::Foundation1),
-            foundation: Foundation::Foundation2,
-        });
-        let recording =
-            SessionRecording::from_instructions_unchecked(7, DrawStockConfig::DrawOne, [bogus]);
-        let v6 = serde_json::json!({
-            "draw_mode": DrawStockConfig::DrawOne,
-            "mode": GameMode::Classic,
-            "elapsed_seconds": 0,
-            "seed": 7,
-            "take_from_foundation": true,
-            "schema_version": 6,
-            "recording": recording,
-        });
-        let err =
-            serde_json::from_value::<GameState>(v6).expect_err("invalid history must be rejected");
-        assert!(err.to_string().contains("invalid"), "got: {err}");
-    }
-
     #[test]
     fn from_recording_of_fresh_deal_returns_empty_instructions() {
         let game = GameState::new(7, DrawStockConfig::DrawThree);

solitaire_data/src/storage.rs

@@ -477,7 +477,7 @@ mod tests {
     /// again must reproduce byte-identical JSON. `undo_count` deliberately resets
     /// to 0 on load because only the forward instruction history is persisted.
     #[test]
-    fn game_state_v6_mid_game_round_trip() {
+    fn game_state_v5_mid_game_round_trip() {
         use solitaire_core::KlondikeInstruction;
         use solitaire_core::game_state::GameState;
 
@@ -518,16 +518,11 @@ mod tests {
 
         save_game_state_to(&path, &gs).expect("save");
 
-        // Verify the file carries the v6 schema marker and the recording.
+        // Verify the file carries the v5 schema marker.
         let json = fs::read_to_string(&path).expect("read json");
-        let parsed: serde_json::Value = serde_json::from_str(&json).expect("parse saved json");
-        assert_eq!(
-            parsed["schema_version"], 6,
-            "saved file must use schema version 6",
-        );
         assert!(
-            parsed["recording"].is_object(),
-            "saved file must embed the session recording",
+            json.contains("\"schema_version\"") && json.contains('5'),
+            "saved file must use schema version 5",
         );
 
         let loaded =
@@ -535,7 +530,7 @@ mod tests {
 
         // The forward instruction history round-trips, so the reconstructed board
         // re-serialises to byte-identical JSON.
-        let path_reload = gs_path("v6_mid_game_reload");
+        let path_reload = gs_path("v5_mid_game_reload");
         let _ = fs::remove_file(&path_reload);
         save_game_state_to(&path_reload, &loaded).expect("re-save loaded");
         assert_eq!(

solitaire_engine/src/win_summary_plugin.rs

@@ -18,7 +18,7 @@
 //! shake duration elapses.
 
 use bevy::prelude::*;
-use solitaire_core::game_state::GameMode;
+use solitaire_core::game_state::{GameMode, GameState};
 use solitaire_core::scoring::compute_time_bonus;
 use solitaire_data::AnimSpeed;
 
@@ -93,6 +93,40 @@ pub struct WinSummaryPending {
     /// score-breakdown reveal can format the mode-multiplier row
     /// (e.g. `Zen ×0.0`, `Classic ×1.0`).
     pub mode: GameMode,
+    /// Per-move-type recap of the winning game, e.g.
+    /// `"21 to foundation · 14 tableau moves · 9 flips · 1 recycle"`.
+    /// Built from the upstream session counters at win time; empty when
+    /// every counter is zero (synthesised test wins).
+    pub move_detail: String,
+}
+
+/// Formats the per-move-type recap line for the win modal from the
+/// upstream session counters. Zero-valued components are omitted so the
+/// line stays short; returns an empty string when nothing was counted
+/// (only possible for synthesised test wins).
+fn build_move_detail(game: &GameState) -> String {
+    let mut parts: Vec<String> = Vec::new();
+    let foundation = game.move_to_foundation_count();
+    if foundation > 0 {
+        parts.push(format!("{foundation} to foundation"));
+    }
+    let tableau = game.move_to_tableau_count();
+    if tableau > 0 {
+        parts.push(format!("{tableau} tableau moves"));
+    }
+    let flips = game.flip_up_count();
+    if flips > 0 {
+        parts.push(format!("{flips} flips"));
+    }
+    let recycles = game.recycle_count();
+    if recycles > 0 {
+        parts.push(format!("{recycles} recycles"));
+    }
+    let returns = game.move_from_foundation_count();
+    if returns > 0 {
+        parts.push(format!("{returns} foundation returns"));
+    }
+    parts.join(" \u{00B7} ")
 }
 
 /// Builds a human-readable XP breakdown string for the win modal.
@@ -492,6 +526,7 @@ fn cache_win_data(
         pending.challenge_level = challenge_level;
         pending.undo_count = game.0.undo_count();
         pending.mode = game.0.mode;
+        pending.move_detail = build_move_detail(&game.0);
 
         if is_new_record {
             toast.write(InfoToastEvent("New Record!".to_string()));
@@ -913,6 +948,18 @@ fn spawn_overlay(
                     ));
                 }
 
+                // Move-type recap (same quiet styling as the XP breakdown)
+                if !pending.move_detail.is_empty() {
+                    card.spawn((
+                        Text::new(pending.move_detail.clone()),
+                        TextFont {
+                            font_size: 15.0,
+                            ..default()
+                        },
+                        TextColor(TEXT_SECONDARY),
+                    ));
+                }
+
                 // Achievements unlocked this game — at most 3 shown explicitly;
                 // excess is summarised with "...and N more".
                 if !session.names.is_empty() {
@@ -1288,6 +1335,49 @@ mod tests {
         assert_eq!(p.mode, GameMode::Classic);
     }
 
+    #[test]
+    fn build_move_detail_fresh_game_is_empty() {
+        let game = GameState::new(42, solitaire_core::DrawStockConfig::DrawOne);
+        assert!(
+            build_move_detail(&game).is_empty(),
+            "no moves yet, so the recap line must be empty"
+        );
+    }
+
+    #[test]
+    fn build_move_detail_reports_played_move_types() {
+        use solitaire_core::KlondikeInstruction;
+        // Drive a real deal forward so the upstream counters accumulate:
+        // prefer foundation moves, then anything else, drawing as needed.
+        let mut game = GameState::new(42, solitaire_core::DrawStockConfig::DrawOne);
+        for _ in 0..80 {
+            let instructions = game.possible_instructions();
+            let next = instructions
+                .iter()
+                .copied()
+                .find(|i| matches!(i, KlondikeInstruction::DstFoundation(_)))
+                .or_else(|| instructions.into_iter().next());
+            match next {
+                Some(i) => {
+                    let _ = game.apply_instruction(i);
+                }
+                None => break,
+            }
+            if game.move_to_foundation_count() > 0 && game.move_to_tableau_count() > 0 {
+                break;
+            }
+        }
+        let detail = build_move_detail(&game);
+        assert!(
+            detail.contains("to foundation"),
+            "seed 42 reaches a foundation move within 80 plies; got: {detail}"
+        );
+        assert!(
+            !detail.contains("0 "),
+            "zero-valued components must be omitted; got: {detail}"
+        );
+    }
+
     #[test]
     fn build_xp_detail_slow_win_with_undo() {
         // 300s >= 120s → no speed bonus; undo used → no no-undo bonus.

solitaire_server/web/replay.js

@@ -315,10 +315,9 @@ btnPlay.addEventListener("click", () => {
     }, STEP_INTERVAL_MS);
 });
 
-/// Step the player back one move. Re-creates the ReplayPlayer and fast-
-/// forwards to (step_idx - 1) without rendering intermediate frames, then
-/// renders once so the CSS transition animates each card to its previous
-/// position.
+/// Step the player back one move via the wasm-side seek (rewinds to the
+/// recorded deal and fast-forwards internally), then renders once so the
+/// CSS transition animates each card to its previous position.
 function stepBack() {
     if (!player || player.step_idx() === 0) return;
     if (playInterval) {
@@ -326,12 +325,7 @@ function stepBack() {
         playInterval = null;
         btnPlay.textContent = "▶ Play";
     }
-    const target = player.step_idx() - 1;
-    player = new ReplayPlayer(replayJson);
-    for (let i = 0; i < target; i++) {
-        player.step();
-    }
-    render(player.state());
+    render(player.seek(player.step_idx() - 1));
     btnPrev.disabled = player.step_idx() === 0;
     btnRestart.disabled = player.step_idx() === 0;
     btnStep.disabled = false;

solitaire_wasm/src/lib.rs

@@ -110,6 +110,9 @@ impl From<&(Card, bool)> for CardSnapshot {
 #[wasm_bindgen]
 pub struct ReplayPlayer {
     game: GameState,
+    /// The recorded deal before any instruction, kept so [`Self::seek_native`]
+    /// can rewind without reparsing the replay JSON.
+    initial: GameState,
     moves: Vec<KlondikeInstruction>,
     step_idx: usize,
 }
@@ -144,12 +147,30 @@ impl ReplayPlayer {
         // the current build maps seeds to deals.
         let (game, moves) = GameState::from_recording(&replay.recording, replay.seed, replay.mode);
         Ok(Self {
+            initial: game.clone(),
             game,
             moves,
             step_idx: 0,
         })
     }
 
+    /// Jump to `step` (clamped to the move count): the board state after
+    /// `step` moves have been applied. Rewinds by resetting to the stored
+    /// initial deal, then fast-forwards — a few hundred instruction
+    /// applications, microseconds in practice.
+    pub fn seek_native(&mut self, step: usize) -> Result<StateSnapshot, MoveError> {
+        let target = step.min(self.moves.len());
+        if target < self.step_idx {
+            self.game = self.initial.clone();
+            self.step_idx = 0;
+        }
+        while self.step_idx < target {
+            self.game.apply_instruction(self.moves[self.step_idx])?;
+            self.step_idx += 1;
+        }
+        Ok(self.snapshot())
+    }
+
     /// Apply the next move. Returns `Ok(None)` once the list is exhausted.
     pub fn step_native(&mut self) -> Result<Option<StateSnapshot>, MoveError> {
         if self.step_idx >= self.moves.len() {
@@ -236,6 +257,25 @@ impl ReplayPlayer {
         }
     }
 
+    /// Jump directly to `step` moves applied (clamped to the move count)
+    /// and return the snapshot there. Backwards seeks rewind to the
+    /// recorded deal and fast-forward, so any position is O(replay length)
+    /// at worst — no JSON reparse, no intermediate renders.
+    ///
+    /// Throws `"replay_desync"` if a recorded move is illegal during the
+    /// fast-forward (corrupt recording).
+    pub fn seek(&mut self, step: usize) -> Result<JsValue, JsValue> {
+        match self.seek_native(step) {
+            Ok(snap) => {
+                serde_wasm_bindgen::to_value(&snap).map_err(|e| JsValue::from_str(&e.to_string()))
+            }
+            Err(e) => {
+                log_replay_move_error(&e);
+                Err(JsValue::from_str("replay_desync"))
+            }
+        }
+    }
+
     /// Total number of moves the replay contains.
     pub fn total_steps(&self) -> usize {
         self.moves.len()
@@ -1062,6 +1102,57 @@ mod tests {
         assert_eq!(orig["stock"], repl["stock"], "stock deal must match");
     }
 
+    /// `seek` must land on exactly the state produced by stepping — both
+    /// forwards (fast-forward from the current position) and backwards
+    /// (rewind to the recorded deal, then fast-forward).
+    #[test]
+    fn seek_matches_stepping_in_both_directions() {
+        let mut game = SolitaireGame {
+            game: GameState::new_with_mode(51, DrawStockConfig::DrawOne, GameMode::Classic),
+        };
+        for _ in 0..24 {
+            let legal_moves = game.legal_moves_native();
+            if legal_moves.is_empty() {
+                break;
+            }
+            let idx = pick_move_index(&legal_moves).unwrap_or_default();
+            game.apply_legal_move_native(idx).expect("advance game");
+        }
+        let replay_json = game
+            .replay_export_native(60, "2026-07-10")
+            .expect("export replay");
+
+        let mut stepped = ReplayPlayer::from_json(&replay_json).expect("player A");
+        let mut seeker = ReplayPlayer::from_json(&replay_json).expect("player B");
+        let total = stepped.total_steps();
+        assert!(total >= 4, "test needs a few moves, got {total}");
+
+        // Forward: step A to k, seek B to k, compare snapshots.
+        let k = total / 2;
+        for _ in 0..k {
+            stepped.step_native().expect("step").expect("mid-replay");
+        }
+        let sought = seeker.seek_native(k).expect("seek forward");
+        assert_eq!(sought, stepped.snapshot(), "forward seek diverged at {k}");
+
+        // Backward: seek B to k - 2 and compare against a fresh stepper.
+        let back = k - 2;
+        let mut fresh = ReplayPlayer::from_json(&replay_json).expect("player C");
+        for _ in 0..back {
+            fresh.step_native().expect("step").expect("mid-replay");
+        }
+        let sought_back = seeker.seek_native(back).expect("seek backward");
+        assert_eq!(
+            sought_back,
+            fresh.snapshot(),
+            "backward seek diverged at {back}"
+        );
+
+        // Clamping: past-the-end seeks stop at the final state.
+        let end = seeker.seek_native(usize::MAX).expect("seek to end");
+        assert_eq!(end.step_idx, total);
+    }
+
     #[test]
     fn debug_api_autonomous_seed_batch_smoke() {
         for seed in 0_u64..128_u64 {