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_engine/src/achievement_plugin.rs

@@ -478,6 +478,31 @@ mod tests {
         );
     }
 
+    #[test]
+    fn no_undo_achievement_does_not_fire_when_undo_was_used() {
+        let mut app = headless_app();
+        // Simulate a win where the player used undo at least once.
+        app.world_mut()
+            .resource_mut::<GameStateResource>()
+            .0
+            .undo_count = 1;
+
+        app.world_mut().send_event(GameWonEvent {
+            score: 1000,
+            time_seconds: 300,
+        });
+        app.update();
+
+        // "no_undo" awards BonusXp(25). If undo was used it must NOT fire.
+        let events = app.world().resource::<Events<XpAwardedEvent>>();
+        let mut cursor = events.get_cursor();
+        let xp_events: Vec<u64> = cursor.read(events).map(|e| e.amount).collect();
+        assert!(
+            !xp_events.contains(&25),
+            "BonusXp(25) must not fire when undo_count > 0; got {xp_events:?}"
+        );
+    }
+
     fn press(app: &mut App, key: KeyCode) {
         let mut input = app.world_mut().resource_mut::<ButtonInput<KeyCode>>();
         input.release(key);