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/rules.rs

@@ -21,7 +21,8 @@ pub fn can_place_on_tableau(card: &Card, pile: &Pile) -> bool {
     match pile.cards.last() {
         None => card.rank.value() == 13,
         Some(top) => {
-            card.rank.value() + 1 == top.rank.value()
+            top.face_up
+                && card.rank.value() + 1 == top.rank.value()
                 && card.suit.is_red() != top.suit.is_red()
         }
     }
@@ -152,4 +153,14 @@ mod tests {
         let p = pile_with(PileType::Tableau(0), vec![card(Suit::Spades, Rank::Nine)]);
         assert!(!can_place_on_tableau(&c, &p));
     }
+
+    #[test]
+    fn tableau_face_down_destination_top_is_invalid() {
+        // A face-down top card must never be a valid placement target.
+        let c = card(Suit::Hearts, Rank::Nine);
+        let mut top = card(Suit::Spades, Rank::Ten);
+        top.face_up = false;
+        let p = pile_with(PileType::Tableau(0), vec![top]);
+        assert!(!can_place_on_tableau(&c, &p));
+    }
 }