feat(data): add Replay::win_move_index for the WIN MOVE scrub marker

First finite step toward the B-2 replay screen-takeover redesign:
the data foundation. Adds an additive optional `win_move_index:
Option<usize>` field on `Replay`, defaulting to `None` via
`#[serde(default)]` so older `latest_replay.json` /
`replays.json` files load unchanged — no `REPLAY_SCHEMA_VERSION`
bump needed since the field is purely additive and nullable.

Populated at the live recording site (`game_plugin::handle_game_won`)
via a new builder-style setter `Replay::with_win_move_index`. For
fresh recordings the value is always `Some(moves.len() - 1)`
because recording freezes on win, but storing the index
explicitly lets the playback UI read the WIN MOVE position
directly without re-deriving it on every render — and leaves
room for future recording semantics that capture post-win state.

UI consumption (the WIN MOVE marker on the scrub bar, plus the
broader screen-takeover redesign — move-log scroller, mini-
tableau preview, playback controls) lands in subsequent commits.

Test coverage: default value, builder set / set-None, on-disk
round-trip, and the legacy-JSON-loads-with-None backward-compat
contract (the test that pins the no-schema-bump claim).

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

solitaire_engine/src/game_plugin.rs

@@ -936,6 +936,11 @@ pub fn record_replay_on_win(
         if recording.moves.is_empty() {
             continue;
         }
+        // Recording freezes on win, so the move that triggered the
+        // win condition is the last one in the list. Storing the
+        // index explicitly lets the playback UI read the WIN MOVE
+        // position directly instead of re-deriving it on every render.
+        let win_move_index = recording.moves.len().checked_sub(1);
         let replay = Replay::new(
             game.0.seed,
             game.0.draw_mode.clone(),
@@ -944,7 +949,8 @@ pub fn record_replay_on_win(
             ev.score,
             Utc::now().date_naive(),
             recording.moves.clone(),
-        );
+        )
+        .with_win_move_index(win_move_index);
         let Some(p) = path.as_ref().and_then(|r| r.0.as_deref()) else {
             // No persistence path configured (e.g. tests / minimal Linux
             // containers without dirs::data_dir). The in-memory replay