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/progress_plugin.rs

@@ -244,4 +244,25 @@ mod tests {
         assert_eq!(fired.len(), 1);
         assert_eq!(fired[0].total_xp, total_xp);
     }
+
+    #[test]
+    fn zen_mode_win_awards_base_xp() {
+        // Zen mode suppresses score display but XP is still awarded normally.
+        // score=0 in the event (Zen keeps score at 0), time=300 (no speed bonus),
+        // undo_count=0 so no-undo bonus applies: expected 50+25=75.
+        let mut app = headless_app();
+        app.world_mut()
+            .resource_mut::<GameStateResource>()
+            .0
+            .mode = solitaire_core::game_state::GameMode::Zen;
+
+        app.world_mut().send_event(GameWonEvent {
+            score: 0,        // Zen mode keeps score at 0
+            time_seconds: 300,
+        });
+        app.update();
+
+        let xp = app.world().resource::<ProgressResource>().0.total_xp;
+        assert_eq!(xp, 75, "Zen win: base 50 + no-undo 25 = 75");
+    }
 }