Implement PSP audio playback via native sceAudio

Reserves a hardware channel per stream (64-aligned sample count,
mono/stereo format) and outputs through sceAudioOutputPannedBlocking,
giving PSP real stereo panning from directionality. Enforces the
44100Hz hardware-channel constraint with a clear error instead of
silently mispitching, and drops the shared test tone to 44100Hz to
match. Verified via the dusk-psp toolchain and a headless PPSSPP run
(correct channel reservation, onEnd firing once, no errors).

src/duskpsp/audio/audiostreampsp.h

@@ -11,7 +11,12 @@
 typedef struct audiostream_s audiostream_t;
 
 typedef struct {
+  // Reserved hardware output channel, from sceAudioChReserve.
   int channel;
+
+  // Number of samples (frames) per output call the channel was reserved
+  // with. 64-aligned, between PSP_AUDIO_SAMPLE_MIN and PSP_AUDIO_SAMPLE_MAX.
+  int samples;
 } audiostreampsp_t;
 
 /**