[Feature] Audio mixer for multiple simultaneous sounds

[vmarcella]

Overview

Implement an audio mixer that combines multiple sound instances into a single
output stream, enabling games to play music, sound effects, and UI sounds
simultaneously.

Current State

Requires basic sound playback. Current design may only support one sound at
a time.

Scope

Goals:

• Mix multiple sound instances together
• Support configurable number of simultaneous sounds
• Per-instance volume applied before mixing
• Master volume applied after mixing

Non-Goals:

• Audio buses/groups (separate issue)
• Effects sends (separate issue)
• Ducking/priority system (separate issue)

Proposed API

pub struct AudioMixer {
  max_voices: usize,
  master_volume: f32,
}

pub struct AudioMixerBuilder {
  // configuration
}

impl AudioMixerBuilder {
  pub fn new() -> Self;
  pub fn with_max_voices(self, count: usize) -> Self;
  pub fn build(self) -> AudioMixer;
}

impl AudioMixer {
  pub fn play(&mut self, buffer: &SoundBuffer) -> Option<SoundInstance>;
  pub fn active_voice_count(&self) -> usize;
}

Acceptance Criteria

• Play at least 32 sounds simultaneously
• Sounds mix without clipping (normalize or limit)
• Finished sounds free their voice slot
• Return None when all voices are in use
• Example playing multiple overlapping sounds

Affected Crates

lambda-rs, lambda-rs-platform

Notes

• Consider voice stealing for when limit is reached
• Sum samples and apply soft limiting to prevent clipping