Cache PSP volume/pan computation, mix channel + master volume into mixer playback, wire boa.mp3 as looping BGM

- PSP output thread now caches the last computed leftVolume/rightVolume and
  only recomputes them when stream->volume/directionality actually changed
  since the previous chunk, instead of recomputing every ~23ms chunk
  unconditionally.
- Mixer channels and the mixer itself now carry their own volume
  (audioMixerSetChannelVolume()/audioMixerSetMasterVolume()), multiplied
  with each sound's own volume every audioMixerUpdateEarly() and applied via
  audioStreamSetVolume() - so changing a channel's or the master volume
  affects whatever's already playing, not just future sounds. Also fixed
  audioMixerUpdateEarly() to tolerate a PLAY command queued outside the
  normal per-frame cycle (no loadingAsset yet from a preceding
  audioMixerUpdateLate(), e.g. during engine startup) by leaving it queued
  for the following frame instead of asserting.
- engine.c now plays boa.mp3 on AUDIO_MIXER_CHANNEL_BGM_0 on loop through
  the mixer instead of driving a raw audiostream_t directly, exercising the
  new mixer pipeline end to end.

Built and verified on Linux, PSP (Docker) and GameCube (Docker).

Co-Authored-By: Claude Sonnet 5 <[email protected]>

src/duskpsp/audio/audiostreampsp.h

@@ -141,6 +141,21 @@ typedef struct {
   // pass's last, possibly-partial chunk.
   bool_t readReachedEnd;
   bool_t readFailed;
+
+  // Cache of the last stream->volume/directionality the output thread
+  // computed lastLeftVolume/lastRightVolume from - recomputing
+  // audioStreamGetPanFactors() and both multiplications is wasted work on
+  // every single chunk (~23ms) whenever neither has actually changed since
+  // the previous one, which is the common case outside of an active fade/
+  // pan. PSP itself has no persistent per-channel volume to cache this
+  // for us (see sceAudioOutputPannedBlocking's own call site) - hence
+  // caching it ourselves. Output-thread-only, like the rest of this
+  // section - see sceAudioOutputPannedBlocking's own call site.
+  float_t lastVolume;
+  float_t lastDirectionality;
+  int lastLeftVolume;
+  int lastRightVolume;
+  bool_t hasLastVolume;
 } audiostreampsp_t;
 
 /**