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>

selfhost/main.ludic

@@ -165,6 +165,13 @@ entry {
     # dead-stripped; -needed_framework forces the load command so the class is
     # registered and #51's gamepad polling can see it.
     cmd = `{cmd} {cocoa} -framework Cocoa -Wl,-needed_framework,GameController -Wl,-rpath,@loader_path`
+    # Audio.* (#22) pulls in the AVAudioPlayer backend and AVFoundation, but only
+    # when the program actually uses it — the snd_* calls are is_windowed()-guarded
+    # yet still live in a windowed build, so the backend must link.
+    if g_uses_audio {
+      let audio = path_join(home, "runtime/native/audio.ll")
+      cmd = `{cmd} {audio} -Wl,-needed_framework,AVFoundation`
+    }
   }
   cmd = `{cmd} -o {out}`