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>

docs/PORT-PLAN.md

@@ -67,6 +67,7 @@ src/host/live_client.cpp                     # run_live_client：40250 客户端
 src/host/vulkan_window.cpp                   # SDL 窗口/事件 + Vulkan 渲染器（回放 Render3DDraw / UIRenderCommand）
 src/host/frame_policy.cpp                    # 帧率策略（FrameRatePolicy）
 src/host/native_audio.cpp                    # MilesLib AudioCommand → SDL 音频流
+src/host/audio_decode.cpp                    # .wav（手写 PCM 解析）/.mp3（dr_mp3）→ 44.1 kHz 立体声 float，全平台同一路径
 src/host/android_perf.h                      # Android 帧率、ADPF、电量/温度（JNI 到 MainActivity）
 android/                                     # Gradle 工程：SDLActivity 外壳 MainActivity、APK 打包
 ```