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_core/src/achievement.rs

@@ -577,4 +577,46 @@ mod tests {
         let ids: Vec<&str> = check_achievements(&c).iter().map(|d| d.id).collect();
         assert!(!ids.contains(&"speed_demon"));
     }
+
+    #[test]
+    fn check_achievements_returns_multiple_when_conditions_met() {
+        // A context where first_win, on_a_roll, and no_undo all trigger at once.
+        let mut c = ctx();
+        c.games_won = 1;
+        c.win_streak_current = 3;
+        c.last_win_used_undo = false;
+        c.last_win_time_seconds = 999;
+
+        let ids: Vec<&str> = check_achievements(&c).iter().map(|d| d.id).collect();
+        assert!(ids.contains(&"first_win"), "first_win should unlock");
+        assert!(ids.contains(&"on_a_roll"), "on_a_roll should unlock");
+        assert!(ids.contains(&"no_undo"), "no_undo should unlock");
+        assert!(ids.len() >= 3, "at least 3 achievements must fire simultaneously");
+    }
+
+    #[test]
+    fn perfectionist_implies_no_undo_both_fire_together() {
+        // perfectionist requires !used_undo && score >= 5000, which is a strict
+        // superset of no_undo's condition. Both must appear in the result.
+        let mut c = ctx();
+        c.games_won = 1;
+        c.last_win_used_undo = false;
+        c.last_win_score = 5_000;
+        c.last_win_time_seconds = 999;
+
+        let ids: Vec<&str> = check_achievements(&c).iter().map(|d| d.id).collect();
+        assert!(ids.contains(&"perfectionist"), "perfectionist must unlock");
+        assert!(ids.contains(&"no_undo"), "no_undo must also unlock when perfectionist does");
+    }
+
+    #[test]
+    fn perfectionist_score_well_above_threshold_still_passes() {
+        let mut c = ctx();
+        c.games_won = 1;
+        c.last_win_used_undo = false;
+        c.last_win_score = 50_000;
+
+        let ids: Vec<&str> = check_achievements(&c).iter().map(|d| d.id).collect();
+        assert!(ids.contains(&"perfectionist"), "score far above threshold must pass");
+    }
 }