Implement SDL2_mixer audio for WASM builds, closes #247

Replace no-op audio stubs in SDL2Types.h with real SDL2_mixer-backed
implementations of SoundBuffer, Sound, and Music. This enables audio
playback in the browser with zero changes to Python bindings.

- Add -sUSE_SDL_MIXER=2 to Emscripten compile/link flags (CMakeLists.txt)
- Initialize Mix_OpenAudio in SDL2Renderer::init(), Mix_CloseAudio in shutdown()
- SoundBuffer: Mix_LoadWAV/Mix_LoadWAV_RW with duration computation
- Sound: channel-based playback with Mix_ChannelFinished tracking
- Music: global channel streaming via Mix_LoadMUS/Mix_PlayMusic
- Volume conversion: SFML 0-100 scale to SDL_mixer 0-128 scale

Known limitations on web: Music.duration and Music.position getters
return 0 (SDL_mixer 2.0.2 lacks Mix_MusicDuration/Mix_GetMusicPosition).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/platform/SDL2Renderer.h

@@ -44,6 +44,7 @@ public:
     void shutdown();
     bool isInitialized() const { return initialized_; }
     bool isGLInitialized() const { return glInitialized_; }
+    bool isAudioInitialized() const { return audioInitialized_; }
 
     // Built-in shader programs
     enum class ShaderType {
@@ -100,6 +101,7 @@ private:
 
     bool initialized_ = false;
     bool glInitialized_ = false;
+    bool audioInitialized_ = false;
 
     // Built-in shader programs
     unsigned int shapeProgram_ = 0;