feat(engine,assetgen): synthesized SFX + kira AudioPlugin

- New solitaire_assetgen crate with gen_sfx binary: synthesizes
  five 44.1kHz mono 16-bit PCM WAVs (flip/place/deal/invalid/fanfare)
  from an LCG noise source + sine/square synths. Output committed
  under assets/audio/.
- AudioPlugin (engine): embeds the WAVs via include_bytes!, decodes
  once with kira::StaticSoundData, plays on Draw / Move / NewGame /
  GameWon events. card_invalid is loaded but unused — wiring it
  needs a MoveRejectedEvent.
- AudioManager kept on the main thread (NonSend) since cpal is !Send
  on some platforms; degrades gracefully if no audio device present.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

solitaire_app/src/main.rs

@@ -1,8 +1,8 @@
 use bevy::prelude::*;
 use solitaire_engine::{
-    AchievementPlugin, AnimationPlugin, CardPlugin, ChallengePlugin, DailyChallengePlugin,
-    GamePlugin, HelpPlugin, InputPlugin, ProgressPlugin, StatsPlugin, TablePlugin, TimeAttackPlugin,
-    WeeklyGoalsPlugin,
+    AchievementPlugin, AnimationPlugin, AudioPlugin, CardPlugin, ChallengePlugin,
+    DailyChallengePlugin, GamePlugin, HelpPlugin, InputPlugin, ProgressPlugin, StatsPlugin,
+    TablePlugin, TimeAttackPlugin, WeeklyGoalsPlugin,
 };
 
 fn main() {
@@ -30,5 +30,6 @@ fn main() {
         .add_plugins(ChallengePlugin)
         .add_plugins(TimeAttackPlugin)
         .add_plugins(HelpPlugin)
+        .add_plugins(AudioPlugin)
         .run();
 }