[Feature] Basic sound playback

[vmarcella]

Overview

Add the ability to play loaded sound buffers through the audio device with
basic transport controls (play, pause, stop).

Current State

Audio device initialization and sound loading must be implemented first
(see related issues). No playback capability exists.

Scope

Goals:

• Play a SoundBuffer through an AudioDevice
• Pause and resume playback
• Stop playback
• Query playback state (playing, paused, stopped)
• Support looping playback

Non-Goals:

• Volume control
• Pitch/speed control
• Spatial audio
• Multiple simultaneous sounds

Proposed API

// crates/lambda-rs/src/audio/mod.rs
pub struct SoundInstance {
  // internal handle
}

impl SoundInstance {
  pub fn play(&mut self);
  pub fn pause(&mut self);
  pub fn stop(&mut self);
  pub fn set_looping(&mut self, looping: bool);
  pub fn is_playing(&self) -> bool;
  pub fn is_paused(&self) -> bool;
}

// On AudioContext or similar
pub fn play_sound(&mut self, buffer: &SoundBuffer) -> SoundInstance;

Acceptance Criteria

• Play a loaded sound buffer to completion
• Pause and resume without audio artifacts
• Stop playback and reset position
• Loop sounds continuously when enabled
• Example demonstrating play/pause/stop

Affected Crates

lambda-rs, lambda-rs-platform

Notes

• Sound instances should be lightweight handles
• Consider ring buffer for audio thread communication
• Audio callback runs on separate thread; avoid blocking