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_app/src/main.rs

@@ -2,9 +2,9 @@ use bevy::prelude::*;
 use solitaire_data::{load_settings_from, provider_for_backend, settings_file_path, Settings};
 use solitaire_engine::{
     AchievementPlugin, AnimationPlugin, AudioPlugin, AutoCompletePlugin, CardPlugin,
-    ChallengePlugin, DailyChallengePlugin, GamePlugin, HelpPlugin, HudPlugin, InputPlugin,
-    LeaderboardPlugin, OnboardingPlugin, PausePlugin, ProgressPlugin, SettingsPlugin, StatsPlugin,
-    SyncPlugin, TablePlugin, TimeAttackPlugin, WeeklyGoalsPlugin,
+    ChallengePlugin, CursorPlugin, DailyChallengePlugin, GamePlugin, HelpPlugin, HudPlugin,
+    InputPlugin, LeaderboardPlugin, OnboardingPlugin, PausePlugin, ProgressPlugin, SettingsPlugin,
+    StatsPlugin, SyncPlugin, TablePlugin, TimeAttackPlugin, WeeklyGoalsPlugin,
 };
 
 fn main() {
@@ -29,6 +29,7 @@ fn main() {
         .add_plugins(GamePlugin)
         .add_plugins(TablePlugin)
         .add_plugins(CardPlugin)
+        .add_plugins(CursorPlugin)
         .add_plugins(InputPlugin)
         .add_plugins(AnimationPlugin)
         .add_plugins(AutoCompletePlugin)