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 <noreply@anthropic.com>

cmake/modules/Findminimp3.cmake

@@ -0,0 +1,28 @@
+# Copyright (c) 2026 Dominic Masters
+#
+# This software is released under the MIT License.
+# https://opensource.org/licenses/MIT
+
+# minimp3 has no tagged releases - pinned to a specific commit instead of a
+# branch for reproducibility.
+include(FetchContent)
+FetchContent_Declare(
+  minimp3
+  GIT_REPOSITORY https://github.com/lieff/minimp3.git
+  GIT_TAG        ea99364f61c14656440e8d77e9c233ccf3124633
+)
+
+FetchContent_MakeAvailable(minimp3)
+
+set(MINIMP3_INCLUDE_DIR "${minimp3_SOURCE_DIR}")
+set(MINIMP3_HEADER "${minimp3_SOURCE_DIR}/minimp3.h")
+
+if(EXISTS "${MINIMP3_HEADER}")
+  add_library(minimp3 INTERFACE)
+  target_include_directories(minimp3 INTERFACE "${MINIMP3_INCLUDE_DIR}")
+  set(minimp3_FOUND TRUE)
+else()
+  set(minimp3_FOUND FALSE)
+endif()
+
+mark_as_advanced(MINIMP3_INCLUDE_DIR MINIMP3_HEADER)