host: cross-platform audio decode (Android has sound)

Replace the macOS-only AudioToolbox decoder with mt_host::decode_audio: a
RIFF/WAVE PCM parser plus vendored dr_mp3, resampled/remixed to 44.1 kHz
stereo float with SDL_ConvertAudioSamples. macOS and Android now share one
path; the AudioToolbox link is gone.

BGM is no longer cached in clips_ (a 5-minute track is ~100 MB of float):
only the playing track is kept, and a new one decodes on a worker thread
(the pack read stays on the main thread) so map changes do not stall.

host.audio_decode test: synthetic wav layouts + all 1578 pack .wav, 9 pack
.mp3 and 25 Client/BGM .mp3 decode (7060 s of audio in ~3.6 s on M2).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/port/CMakeLists.txt

@@ -175,6 +175,7 @@ if(BUILD_TESTING AND CMAKE_SYSTEM_NAME STREQUAL CMAKE_HOST_SYSTEM_NAME)
     if(NOT MT_40250_CLIENT)
         set(MT_40250_CLIENT "${MT_40250_SOURCE}/../../Client")
     endif()
+    set(MT_40250_CLIENT "${MT_40250_CLIENT}" PARENT_SCOPE) # also for src/host tests
     add_executable(port_eterpack_test ${PROJECT_SOURCE_DIR}/tests/port/port_eterpack_test.cpp)
     target_link_libraries(port_eterpack_test PRIVATE port_platform)
     add_test(NAME port.eterpack COMMAND $<TARGET_FILE:port_eterpack_test> ${MT_40250_CLIENT})