feat: audio system with 15 procedurally generated sounds via soundgen

- AudioEngine with rodio (WAV playback, throttling, volume, toggle)
- 15 embedded sounds: jump, shoot, hit, explosion, death, pickup, descend,
  powerup, step, lava_bubble, acid_sizzle, water_splash, fire_crackle,
  ui_click, goblin_growl
- Sound events: shoot, hit, explosion (fireball), pickup, descend, powerup,
  jump, combat hit, ambient material sounds (lava/fire/acid/water)
- M key toggles audio in GPU modes
- Ambient sounds: scans 15-cell radius around player, plays throttled sounds
  for nearby lava/fire/acid/water
- AudioEngine gracefully degrades when no audio device available
- All 185 tests + 14 scenarios pass

Cargo.toml

@@ -15,6 +15,7 @@ ash-window = "0.13"
 raw-window-handle = "0.6"
 bytemuck = { version = "1", features = ["derive"] }
 image = { version = "0.25", default-features = false, features = ["png"] }
+rodio = { version = "0.20", default-features = false, features = ["wav"] }
 
 [profile.release]
 opt-level = 3