feat(replay): wire ← / → keyboard accelerators for paused stepping

→ during a paused replay advances by one move (mirrors the Stop
button's existing forward-step semantics). ← decrements the
cursor and dispatches `UndoRequestEvent`, which the game's
`handle_undo` reads next frame to reverse its most-recent move
— hooking the existing undo system rather than replaying
forward from cursor 0 (every replay-applied move pushes to the
undo stack the same way a player move would, so undo is the
right reversal primitive).

Both accelerators are paused-only — backwards via a new
`step_backwards_replay_playback` in `replay_playback.rs` that
hard-gates with the same destructure pattern as
`step_replay_playback`. Pressing → during running playback or ←
at cursor 0 are silent no-ops; the player learns "pause first,
then arrow."

The mockup labels these `[← →] scrub` (continuous fast scan).
Single-move step is the closest behaviour shippable today —
continuous scrub would need either a key-held event source or
an internal speed-up loop. Footer hint reads
`[← →] step` to match what's wired rather than the aspirational
"scrub."

Footer hint extended in lockstep:
`[SPACE] pause/resume · [ESC] stop · [← →] step` — the
only-wired-keybinds discipline holds.

ReplayOverlayPlugin gains `add_message::<UndoRequestEvent>()`
defensively so the plugin can run under MinimalPlugins without
GamePlugin attached (idempotent registration; harmless when
GamePlugin is also present).

6 new tests (2 hint pins + 4 keyboard scenarios) + 1 helper-pin
update for the new hint string.

Pre-existing flake noted: `daily_challenge_plugin::tests::
check_system_fires_warning_event_only_once_per_day` is failing
because wall-clock UTC is currently within 30 minutes of
midnight, inside the daily-expiry warning window the test
asserts against. Verified pre-existing by stashing all changes
and re-running — failure persists. Same shape as the
`winnable_seed_search` flake the handoff documented earlier
this session: time-dependent, deterministically passes under
different clock conditions. Not introduced by this commit.

Clippy clean.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

solitaire_engine/src/replay_overlay.rs

@@ -28,9 +28,10 @@ use chrono::Datelike;
 
 use crate::font_plugin::FontResource;
 use crate::layout::LayoutResource;
-use crate::events::{DrawRequestEvent, MoveRequestEvent};
+use crate::events::{DrawRequestEvent, MoveRequestEvent, UndoRequestEvent};
 use crate::replay_playback::{
-    step_replay_playback, stop_replay_playback, toggle_pause_replay_playback, ReplayPlaybackState,
+    step_backwards_replay_playback, step_replay_playback, stop_replay_playback,
+    toggle_pause_replay_playback, ReplayPlaybackState,
 };
 use solitaire_data::ReplayMove;
 use crate::ui_modal::{spawn_modal_button, ButtonVariant};
@@ -275,6 +276,7 @@ impl Plugin for ReplayOverlayPlugin {
         // in production (alongside `replay_playback`) is harmless.
         app.add_message::<MoveRequestEvent>()
             .add_message::<DrawRequestEvent>()
+            .add_message::<UndoRequestEvent>()
             .add_systems(
                 Update,
                 (
@@ -288,6 +290,7 @@ impl Plugin for ReplayOverlayPlugin {
                     handle_step_button,
                     handle_pause_keyboard,
                     handle_stop_keyboard,
+                    handle_arrow_keyboard,
                     handle_stop_button,
                 )
                     .chain(),
@@ -786,12 +789,11 @@ fn keybind_footer_mode_text() -> &'static str {
 /// Pure helper — returns the keybind-hint text shown on the right
 /// side of the keybind-hint footer row. Lists only the keys that
 /// are *actually wired* today: the Space accelerator for
-/// pause/resume and the ESC accelerator for stop. Future commits
-/// that wire ← / → for prev/next move will extend this string —
-/// the footer never lists unimplemented keybinds (would lie to
-/// users).
+/// pause/resume, the ESC accelerator for stop, and the ← / →
+/// accelerators for paused single-move stepping. The footer never
+/// lists unimplemented keybinds (would lie to users).
 fn keybind_footer_hint_text() -> &'static str {
-    "[SPACE] pause/resume \u{00B7} [ESC] stop" // · separator
+    "[SPACE] pause/resume \u{00B7} [ESC] stop \u{00B7} [\u{2190}\u{2192}] step" // · separator
 }
 
 /// Pure helper — returns the WIN MOVE marker's left-edge position as
@@ -1094,6 +1096,37 @@ fn handle_pause_keyboard(
     toggle_pause_replay_playback(&mut state);
 }
 
+/// Watches the arrow keys for the paused single-step
+/// accelerators. UI-first contract from CLAUDE.md §3.3 is
+/// satisfied by the on-screen Step button (forward only); these
+/// are the optional accelerators that also surface a backwards
+/// step.
+///
+/// Both keys are paused-only — the underlying step helpers
+/// hard-gate via destructure on `paused: true`. Pressing → during
+/// running playback or ← at cursor 0 are silent no-ops; the
+/// player learns "pause first, then arrow."
+///
+/// The mockup labels these `[← →] scrub` (continuous fast scan).
+/// Single-move step is the closest behaviour shippable today; the
+/// footer hint reads `[← →] step` to match what's wired rather
+/// than the aspirational "scrub."
+fn handle_arrow_keyboard(
+    keys: Option<Res<ButtonInput<KeyCode>>>,
+    mut state: ResMut<ReplayPlaybackState>,
+    mut moves_writer: MessageWriter<MoveRequestEvent>,
+    mut draws_writer: MessageWriter<DrawRequestEvent>,
+    mut undo_writer: MessageWriter<UndoRequestEvent>,
+) {
+    let Some(keys) = keys else { return };
+    if keys.just_pressed(KeyCode::ArrowRight) {
+        step_replay_playback(&mut state, &mut moves_writer, &mut draws_writer);
+    }
+    if keys.just_pressed(KeyCode::ArrowLeft) {
+        step_backwards_replay_playback(&mut state, &mut undo_writer);
+    }
+}
+
 /// Watches `Esc` for the keyboard stop accelerator. UI-first
 /// contract from CLAUDE.md §3.3 is satisfied by the on-screen
 /// Stop button; this is the optional accelerator.
@@ -1958,8 +1991,8 @@ mod tests {
         );
         assert_eq!(
             keybind_footer_hint_text(),
-            "[SPACE] pause/resume \u{00B7} [ESC] stop",
-            "hint text must list both wired keybinds (Space → pause/resume, Esc → stop) separated by a middle dot",
+            "[SPACE] pause/resume \u{00B7} [ESC] stop \u{00B7} [\u{2190}\u{2192}] step",
+            "hint text must list all three wired keybind groups (Space → pause/resume, Esc → stop, ←→ → step) separated by middle dots",
         );
     }
 
@@ -2313,6 +2346,153 @@ mod tests {
         );
     }
 
+    /// Hint must also list the arrow-key step accelerators.
+    /// Pinned separately from the Space + Esc test so a future
+    /// regression that drops only the arrows is caught here even
+    /// if the Space + Esc check still passes.
+    #[test]
+    fn keybind_footer_hint_lists_arrow_steps() {
+        let hint = keybind_footer_hint_text();
+        assert!(
+            hint.contains("\u{2190}\u{2192}"),
+            "hint must surface the ←→ step accelerators; got {hint:?}",
+        );
+        assert!(
+            hint.contains("step"),
+            "hint must label the arrow accelerators as 'step' \
+             (matches what's wired — single-move step, not continuous scrub); got {hint:?}",
+        );
+    }
+
+    /// Pressing → while paused advances the cursor by exactly one
+    /// — same end-state as clicking the on-screen Step button.
+    #[test]
+    fn arrow_right_keyboard_advances_cursor_while_paused() {
+        let mut app = headless_app();
+        app.init_resource::<ButtonInput<KeyCode>>();
+        set_state(&mut app, pressed_paused_state(5, 0));
+        app.update();
+
+        app.world_mut()
+            .resource_mut::<ButtonInput<KeyCode>>()
+            .press(KeyCode::ArrowRight);
+        app.update();
+
+        match app.world().resource::<ReplayPlaybackState>() {
+            ReplayPlaybackState::Playing { cursor, paused, .. } => {
+                assert_eq!(
+                    *cursor, 1,
+                    "→ must advance the cursor by exactly one while paused",
+                );
+                assert!(
+                    *paused,
+                    "→ must leave the paused flag untouched",
+                );
+            }
+            other => panic!("expected Playing, got {other:?}"),
+        }
+    }
+
+    /// Pressing → while running is a no-op — the existing
+    /// `step_replay_playback` guard prevents racing the tick loop.
+    #[test]
+    fn arrow_right_keyboard_is_noop_while_running() {
+        let mut app = headless_app();
+        app.init_resource::<ButtonInput<KeyCode>>();
+        set_state(&mut app, running_state(5, 0));
+        app.update();
+
+        app.world_mut()
+            .resource_mut::<ButtonInput<KeyCode>>()
+            .press(KeyCode::ArrowRight);
+        app.update();
+
+        match app.world().resource::<ReplayPlaybackState>() {
+            ReplayPlaybackState::Playing { cursor, paused, .. } => {
+                assert_eq!(*cursor, 0, "→ must not race the tick loop");
+                assert!(!*paused);
+            }
+            other => panic!("expected Playing, got {other:?}"),
+        }
+    }
+
+    /// Pressing ← while paused with cursor > 0 decrements the
+    /// cursor by exactly one. The corresponding game-state reversal
+    /// happens when `handle_undo` reads the dispatched
+    /// `UndoRequestEvent` — that's covered in the playback core's
+    /// integration test, not here.
+    #[test]
+    fn arrow_left_keyboard_decrements_cursor_while_paused() {
+        let mut app = headless_app();
+        app.init_resource::<ButtonInput<KeyCode>>();
+        // Start paused at cursor=3 so there's room to step backwards.
+        set_state(&mut app, pressed_paused_state(5, 3));
+        app.update();
+
+        app.world_mut()
+            .resource_mut::<ButtonInput<KeyCode>>()
+            .press(KeyCode::ArrowLeft);
+        app.update();
+
+        match app.world().resource::<ReplayPlaybackState>() {
+            ReplayPlaybackState::Playing { cursor, paused, .. } => {
+                assert_eq!(
+                    *cursor, 2,
+                    "← must decrement the cursor by exactly one while paused",
+                );
+                assert!(
+                    *paused,
+                    "← must leave the paused flag untouched",
+                );
+            }
+            other => panic!("expected Playing, got {other:?}"),
+        }
+    }
+
+    /// Pressing ← at cursor 0 is a no-op (nothing to rewind past).
+    #[test]
+    fn arrow_left_keyboard_is_noop_at_cursor_zero() {
+        let mut app = headless_app();
+        app.init_resource::<ButtonInput<KeyCode>>();
+        set_state(&mut app, pressed_paused_state(5, 0));
+        app.update();
+
+        app.world_mut()
+            .resource_mut::<ButtonInput<KeyCode>>()
+            .press(KeyCode::ArrowLeft);
+        app.update();
+
+        match app.world().resource::<ReplayPlaybackState>() {
+            ReplayPlaybackState::Playing { cursor, .. } => {
+                assert_eq!(*cursor, 0, "← at cursor 0 must be a no-op");
+            }
+            other => panic!("expected Playing, got {other:?}"),
+        }
+    }
+
+    /// Pressing ← while running is a no-op — same hard-gate
+    /// rationale as the forward-step paused-only check.
+    #[test]
+    fn arrow_left_keyboard_is_noop_while_running() {
+        let mut app = headless_app();
+        app.init_resource::<ButtonInput<KeyCode>>();
+        set_state(&mut app, running_state(5, 3));
+        app.update();
+
+        app.world_mut()
+            .resource_mut::<ButtonInput<KeyCode>>()
+            .press(KeyCode::ArrowLeft);
+        app.update();
+
+        match app.world().resource::<ReplayPlaybackState>() {
+            ReplayPlaybackState::Playing { cursor, paused, .. } => {
+                assert_eq!(*cursor, 3, "← must not race the tick loop");
+                assert!(!*paused);
+            }
+            other => panic!("expected Playing, got {other:?}"),
+        }
+    }
+
     #[test]
     fn space_keyboard_toggles_paused_flag() {
         let mut app = headless_app();

solitaire_engine/src/replay_playback.rs

@@ -42,7 +42,7 @@
 use bevy::prelude::*;
 use solitaire_data::{Replay, ReplayMove};
 
-use crate::events::{DrawRequestEvent, MoveRequestEvent, StateChangedEvent};
+use crate::events::{DrawRequestEvent, MoveRequestEvent, StateChangedEvent, UndoRequestEvent};
 use crate::game_plugin::{GameMutation, RecordingReplay};
 use crate::resources::GameStateResource;
 use crate::settings_plugin::SettingsResource;
@@ -284,6 +284,52 @@ pub fn step_replay_playback(
     true
 }
 
+/// Steps the replay **backwards** by exactly one move while paused.
+///
+/// Strategy: the live game's undo system is the source of truth for
+/// reversing moves. Every move the replay forward-stepped (via
+/// [`step_replay_playback`] or the auto-advance loop in
+/// [`tick_replay_playback`]) was dispatched as a canonical
+/// [`MoveRequestEvent`] / [`DrawRequestEvent`], which the game
+/// applied and pushed onto its undo stack. So a backwards step here
+/// is simply: decrement the cursor (so the about-to-apply move
+/// re-points at the one we're rewinding past) and fire an
+/// [`UndoRequestEvent`] so the game reverses its most-recent move
+/// next frame.
+///
+/// Hard-gated to the paused state via destructure pattern —
+/// matches the existing [`step_replay_playback`] gate so the
+/// player can only scrub one direction at a time and the tick
+/// loop never races a manual rewind.
+///
+/// Returns `false` and is a no-op in three cases:
+/// - State isn't `Playing` (no replay attached).
+/// - State is `Playing` but not paused (the tick loop owns the cursor).
+/// - Cursor is already at 0 (nothing to rewind past).
+///
+/// Returns `true` on a successful step; the actual game-state
+/// reversal happens next frame when `handle_undo` reads the
+/// `UndoRequestEvent`.
+pub fn step_backwards_replay_playback(
+    state: &mut ResMut<ReplayPlaybackState>,
+    undo_writer: &mut MessageWriter<UndoRequestEvent>,
+) -> bool {
+    let ReplayPlaybackState::Playing {
+        cursor,
+        paused: true,
+        ..
+    } = state.as_mut()
+    else {
+        return false;
+    };
+    if *cursor == 0 {
+        return false;
+    }
+    *cursor -= 1;
+    undo_writer.write(UndoRequestEvent);
+    true
+}
+
 /// Tick system. Runs every frame; only does work when
 /// [`ReplayPlaybackState::is_playing`].
 ///