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/duskdolphin/audio/CMakeLists.txt

@@ -9,3 +9,21 @@ target_sources(${DUSK_LIBRARY_TARGET_NAME}
     audiodolphin.c
     audiostreamdolphin.c
 )
+
+# No hardware MP3 decoder used on Dolphin (see audiostreammp3decoder.h's
+# own comment on why libogc's MP3Player wrapper isn't a fit) - use the
+# same shared minimp3-based software backend as Linux, sourced directly
+# from src/dusk/audio rather than through its own unconditional
+# CMakeLists.txt so platforms with a hardware decoder (PSP) never pull
+# minimp3 in at all.
+if(NOT minimp3_FOUND)
+  find_package(minimp3 REQUIRED)
+endif()
+# PRIVATE to match this toolchain's own convention (see cmake/targets/
+# dolphin.cmake) of avoiding PUBLIC library visibility, which has tripped
+# up the PPC linker for other dependencies in the past.
+target_link_libraries(${DUSK_LIBRARY_TARGET_NAME} PRIVATE minimp3)
+target_sources(${DUSK_LIBRARY_TARGET_NAME}
+  PUBLIC
+    "${DUSK_SOURCES_DIR}/dusk/audio/audiostreammp3decodersw.c"
+)