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>

Cargo.toml

@@ -7,6 +7,7 @@ members = [
     "solitaire_server",
     "solitaire_gpgs",
     "solitaire_app",
+    "solitaire_assetgen",
 ]
 resolver = "2"