feat(engine): "Watch replay" affordance in Stats overlay

The Stats screen now shows the most recent winning replay's caption
("M:SS win on YYYY-MM-DD") and a Watch Replay button. Until the web
viewer is fully wired the click fires a toast pointing the player at
the upcoming `<server>/replays/<id>` URL; once the upload + page
ship the toast is replaced with an actual link.

- New resources `LatestReplayResource(Option<Replay>)` and
  `LatestReplayPath(Option<PathBuf>)` populated at plugin build time
  from the platform-default `latest_replay.json`. Headless mode
  disables I/O the same way `StatsResource` does.
- `refresh_latest_replay_on_win` re-loads from disk after every
  `GameWonEvent` so opening the modal a second time reflects the
  most recent victory rather than a stale snapshot.
- `format_replay_caption` is a pure helper exposed for both the
  Stats button label and (later) toast messaging.
- `WatchReplayButton` marker added to `solitaire_engine`'s public
  re-exports so the future web-side click integrations can match.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

solitaire_engine/src/lib.rs

@@ -122,7 +122,10 @@ pub use selection_plugin::{
     KeyboardDragState, SelectionHighlight, SelectionPlugin, SelectionState,
 };
 pub use splash_plugin::{SplashAge, SplashPlugin, SplashRoot};
-pub use stats_plugin::{StatsPlugin, StatsResource, StatsScreen, StatsUpdate};
+pub use stats_plugin::{
+    format_replay_caption, LatestReplayPath, LatestReplayResource, StatsPlugin, StatsResource,
+    StatsScreen, StatsUpdate, WatchReplayButton,
+};
 pub use sync_plugin::{SyncPlugin, SyncProviderResource};
 pub use ui_focus::{Disabled, FocusGroup, Focusable, FocusedButton, UiFocusPlugin};
 pub use ui_modal::{