feat(audio): Audio.* standard library over a native AVAudioPlayer backend (#22)

Adds the Audio.* namespace and its platform backend, the audio subsystem #22 was
blocked on.

- runtime/native/audio.ll: the macOS backend, AVAudioPlayer driven through the
  objc runtime C ABI (no ObjC/C source), same style as cocoa.ll — snd_load /
  play / stop / playing / set_volume / set_rate. Spliced and linked with
  AVFoundation only when a windowed build actually uses Audio.* (needed_framework,
  since AVAudioPlayer is reached by name).
- runtime/native/audio.ludic: the Audio.* runtime — a handle table, master
  volume/pitch, a single music channel. load/play/play_sound/play_music/stop/
  stop_music/stop_all/volume/pitch/is_playing. Every native call is
  is_windowed()-guarded, so a headless build carries the API as no-ops (load
  returns 0, is_playing false) and needs no audio device.
- compiler: Audio.* namespace dispatch, g_uses_audio splice, snd_* intrinsics +
  declarations, and the conditional AVFoundation link in both the canonical
  (main.ludic) and dev-runner (x app) paths.
- docs: a full docs/language/audio section (10 method pages); check-impl green.
- test: examples/library/audio.ludic self-asserts the headless no-op path.

Playback is out-of-band and never feeds the deterministic sim, but triggers are
frame-driven so replays fire the same sounds. Reseeded; suites green (80 + 29).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/language/audio/_section.md

@@ -0,0 +1,7 @@
+---
+id: audio
+title: Audio
+order: 7
+---
+
+Sound effects and music. A sound is loaded once with <a href="audio-load.html"><code>Audio.load</code></a> into a handle; <a href="audio-play.html"><code>Audio.play</code></a> fires it one-shot and <a href="audio-play_music.html"><code>Audio.play_music</code></a> loops it on a single music channel. Playback is <strong>out-of-band</strong> — the audio device is real-time, not part of the deterministic simulation — but every trigger here is an ordinary frame-driven call, so a recorded run replays the same sounds at the same frames. Headless builds carry the whole API as no-ops (no audio device needed), so the same game code runs under the test harness.