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_engine/src/lib.rs

@@ -2,6 +2,7 @@
 
 pub mod achievement_plugin;
 pub mod animation_plugin;
+pub mod audio_plugin;
 pub mod card_plugin;
 pub mod challenge_plugin;
 pub mod daily_challenge_plugin;
@@ -27,6 +28,7 @@ pub use daily_challenge_plugin::{
 pub use progress_plugin::{LevelUpEvent, ProgressPlugin, ProgressResource, ProgressUpdate};
 pub use weekly_goals_plugin::{WeeklyGoalCompletedEvent, WeeklyGoalsPlugin};
 pub use animation_plugin::{AnimationPlugin, CardAnim};
+pub use audio_plugin::{AudioPlugin, AudioState, SoundLibrary};
 pub use card_plugin::{CardEntity, CardLabel, CardPlugin};
 pub use events::{
     AchievementUnlockedEvent, CardFlippedEvent, DrawRequestEvent, GameWonEvent, MoveRequestEvent,