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_server/src/challenge.rs

@@ -194,4 +194,24 @@ mod tests {
         assert!(g.target_score.is_none());
         assert!(g.max_time_secs.is_none());
     }
+
+    #[test]
+    fn generate_goal_all_variants_have_sane_ranges() {
+        for variant_idx in 0u64..6 {
+            let g = generate_goal("2026-04-26", variant_idx);
+            assert!(!g.description.is_empty(), "variant {variant_idx}: description must not be empty");
+            if let Some(t) = g.max_time_secs {
+                assert!(
+                    (60..=3600).contains(&t),
+                    "variant {variant_idx}: max_time_secs {t} outside [60, 3600]"
+                );
+            }
+            if let Some(s) = g.target_score {
+                assert!(
+                    (1_000..=10_000).contains(&s),
+                    "variant {variant_idx}: target_score {s} outside [1000, 10000]"
+                );
+            }
+        }
+    }
 }