Add MP3 audio stream support with hardware/software decoder backends

New ASSET_LOADER_TYPE_MP3 (hand-rolled MPEG-1/2/2.5 Layer III header
parser - no third-party dependency needed just for metadata, since PSP's
hardware path doesn't need one at all) plus a shared audiostreammp3.c
stream layer mirroring audiostreampcm.c's shape. Generalized the stream
dispatch (hoisted sampleRate/channels onto audiostream_t, added
audioStreamGetTotalFrames()/Seek()/Read()) so all three platform audio
backends keep working unchanged, just calling the generic names instead
of PCM-specific ones.

Two decoder backends behind one interface: PSP uses the real sceMp3
hardware decoder (firmware-offloaded, lazily initialized on first use);
Linux and Dolphin share one minimp3-based software decoder (public
domain, vendored via CMake FetchContent) - libogc's own MP3Player wraps
libmad (GPL) and drives its own output pipeline, not a fit for the
ansnd-based architecture already in place, so skipped in favor of the
shared minimp3 path.

Fixed three real bugs found via hardware/runtime testing along the way:
- LAME's Xing header counts its own placeholder frame in the declared
  total, which made playback stall permanently one frame short of the
  declared end (looked like "never loops") - fixed by subtracting it.
- sceMp3Decode() can return more PCM than one MPEG frame's worth in a
  single call (PSP's pcmBuf is provisioned for 2x), overflowing the
  shared per-frame decode buffer with no bound check - very intermittent
  corruption/clicking on real hardware. Widened the buffer to the real
  worst case and added an assertion.
- sceMp3ResetPlayPosition()'s exact internal reset semantics aren't
  documented precisely enough to trust for looping - occasionally
  disagreed with the fresh stream position fed right after, clicking at
  the loop boundary about 1 in 3-4 loops. Rewind now fully tears down and
  recreates the decoder instead, the same path already proven correct at
  first Init. Also widened the PSP ring buffer to absorb that now-heavier
  operation, capping each top-up call's own work so the bigger buffer
  doesn't turn into one long blocking decode burst instead.

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

src/duskpsp/audio/audiostreampsp.h

@@ -16,13 +16,32 @@ typedef struct audiostream_s audiostream_t;
 // Physical capacity of the ring; must comfortably exceed
 // AUDIO_PSP_LEAD_FRAMES plus one top-up step so a single top-up call can
 // never overwrite data the output thread hasn't consumed yet.
-#define AUDIO_PSP_RING_FRAMES 16384
+//
+// Sized well above what a PCM stream would ever need on its own, because
+// an MP3 stream's loop-wrap (audioStreamMp3DecoderRewind() on PSP) fully
+// tears down and recreates the hardware decoder - a real handle release/
+// reserve round-trip plus a fresh ~16KB Memory Stick read to re-prime
+// sceMp3's stream buffer - all synchronously, inside the same TopUp() call
+// that's supposed to be keeping this ring fed. That's slow enough,
+// occasionally, to eat into a tighter lead margin and click right at the
+// loop point (confirmed on real hardware) - this gives it generous room to
+// do so without the output thread ever catching up to empty.
+#define AUDIO_PSP_RING_FRAMES 49152
 
 // Top-up threshold - audioStreamPSPTopUp() (called once per engine
 // Update(), see audioStreamPSPIsFinished()) only reads more data once the
 // ring drops below this, same trigger dusklinux uses for its own lead
-// margin.
-#define AUDIO_PSP_LEAD_FRAMES 4096
+// margin. See AUDIO_PSP_RING_FRAMES's own comment for why this is sized
+// well beyond dusklinux's equivalent.
+#define AUDIO_PSP_LEAD_FRAMES 12288
+
+// Max frames audioStreamPSPTopUp() will read in one call, regardless of
+// how much ring room is actually free - keeps each call's worst-case
+// blocking time on the main thread bounded and roughly constant even
+// though AUDIO_PSP_RING_FRAMES/AUDIO_PSP_LEAD_FRAMES are large; reaching
+// LEAD from an empty ring just takes a few calls (a few engine frames)
+// instead of one long one. See AUDIO_PSP_RING_FRAMES's own comment.
+#define AUDIO_PSP_TOPUP_STEP_FRAMES 4096
 
 // Max number of pending loop-wrap events (see audiostreampsploopmarker_t
 // below) the ring can remember at once. Sized generously relative to how