feat(engine): playability improvements — input intelligence, audio, HUD, onboarding (#27–#30, #37, #39–#40, #44, #48–#49)

Task #27: Double-click auto-move — best_destination() finds optimal target
(foundation over tableau); handle_double_click() fires MoveRequestEvent.

Task #28: Hint system — find_hint() returns first legal from/to/count triple;
H key tints the source stack HintHighlight (yellow pulse via tick_hint_highlight).

Task #29: No-moves detection — has_legal_moves() checks stock/waste/all face-up
cards; check_no_moves system fires InfoToastEvent("No moves available") once per
stalemate (debounced so it fires only once until the state changes).

Task #30: Forfeit — G key fires ForfeitEvent; StatsPlugin records abandoned game,
persists stats, starts a new deal.

Task #37: Mute-all (M) and mute-music (Shift+M) toggles; MuteState resource
applied in apply_volume_on_change.

Task #39: Daily challenge HUD constraint label (time limit / target score).

Task #40: Undo-count HUD label; amber colour when undos > 0.

Task #44: Win-streak and level line on pause screen.

Task #48: Undo sound routes UndoRequestEvent → lib.flip audio channel.

Task #49: Onboarding banner rich-text key highlights — D and H rendered as
orange KeyHighlightSpan children so they stand out from body text.

Also registers CursorPlugin in solitaire_app (tasks #31/#32 wire-up).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

solitaire_data/src/stats.rs

@@ -152,4 +152,30 @@ mod tests {
         assert_eq!(s.draw_one_wins, 1);
         assert_eq!(s.draw_three_wins, 1);
     }
+
+    #[test]
+    fn win_streak_best_never_decreases_after_shorter_subsequent_streak() {
+        let mut s = StatsSnapshot::default();
+        // Build a streak of 5.
+        for _ in 0..5 {
+            s.update_on_win(100, 60, &DrawMode::DrawOne);
+        }
+        assert_eq!(s.win_streak_best, 5);
+        // Lose (abandon), resetting current.
+        s.record_abandoned();
+        assert_eq!(s.win_streak_current, 0);
+        assert_eq!(s.win_streak_best, 5, "best must survive the loss");
+        // Win once — current becomes 1, best must remain 5.
+        s.update_on_win(100, 60, &DrawMode::DrawOne);
+        assert_eq!(s.win_streak_current, 1);
+        assert_eq!(s.win_streak_best, 5, "best must not drop to match shorter streak");
+    }
+
+    #[test]
+    fn lifetime_score_saturates_at_u64_max() {
+        let mut s = StatsSnapshot::default();
+        s.lifetime_score = u64::MAX - 100;
+        s.update_on_win(200, 60, &DrawMode::DrawOne);
+        assert_eq!(s.lifetime_score, u64::MAX, "lifetime_score must saturate, not overflow");
+    }
 }