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>
