add: implement audio mixing functionality with dynamic layer management

[1Lucas1apk]

Changes

• Added full audio mixing system with dynamic layer management.
• Introduced new REST endpoints:
  • POST /sessions/{sessionId}/players/{guildId}/mix – create a new mix layer.
  • GET /sessions/{sessionId}/players/{guildId}/mix – list all mix layers.
  • PATCH /sessions/{sessionId}/players/{guildId}/mix/{mixId} – update mix volume.
  • DELETE /sessions/{sessionId}/players/{guildId}/mix/{mixId} – remove a mix layer.
• Implemented AudioMixer class with:
  • Independent PCM mixing per layer
  • Volume scaling
  • Automatic layer cleanup
  • Finish detection via empty reads
  • Error handling and emissions (mixStarted, mixEnded, mixError)
• Added mix configuration under mix { enabled, defaultVolume, maxLayersMix, autoCleanup }.
• Added cluster-aware mix command handling through playerManager.
• Player now initializes AudioMixer and emits gateway events:
  • MixStartedEvent
  • MixEndedEvent
• Mixing is now integrated into playback pipeline through streamProcessor.
• Added version bump to 3.3.0.

Why

This adds native support for layered audio playback — allowing bots to play sound effects, transitions, stingers, background loops and dynamic overlays at the same time as the main track.
The system is fully dynamic, cluster-safe, and supports real-time volume adjustments and automatic cleanup.

Checkmarks

• The modified endpoints have been tested.
• Used the same indentation as the rest of the project.
• Still compatible with Lavalink clients.

Additional information

• Mixing is done at PCM level for maximum performance.
• Layer streaming is non-blocking and isolated.
• Default config allows 5 simultaneous layers, adjustable via config.default.js.