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_assetgen/Cargo.toml

@@ -0,0 +1,12 @@
+[package]
+name = "solitaire_assetgen"
+version.workspace = true
+edition.workspace = true
+publish = false
+
+# Dev-only utility: synthesizes placeholder SFX WAV files into `assets/audio/`.
+# Not depended on by any other workspace crate.
+
+[[bin]]
+name = "gen_sfx"
+path = "src/bin/gen_sfx.rs"