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/backend/emit_head.ludic

@@ -67,6 +67,12 @@ function emit_header() -> void {
   emith("declare void @win_mouse(ptr)\n")
   emith("declare void @win_pad(ptr)\n")
   emith("declare void @win_touch(ptr)\n")
+  emith("declare ptr @snd_load(ptr)\n")               # #22 audio backend (audio.ll)
+  emith("declare void @snd_play(ptr, i32, i32, i32)\n")
+  emith("declare void @snd_stop(ptr)\n")
+  emith("declare i32 @snd_playing(ptr)\n")
+  emith("declare void @snd_set_volume(ptr, i32)\n")
+  emith("declare void @snd_set_rate(ptr, i32)\n")
   emith("@__stderrp = external global ptr\n")
   emith("@__stdoutp = external global ptr\n")
   emith("@.fmt_int = private unnamed_addr constant [4 x i8] c\"%d\\0A\\00\"\n")