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>

tools/x/build.ludic

@@ -77,7 +77,9 @@ function cmd_app() -> int {
   let ll = `{outbin}.ll`
   print(`ludicc: {src} -> {outbin}  (Ludic-written compiler -> LLVM IR -> windowed binary)`)
   if not shq(`bin/ludicc --windowed {src} > {ll}`) { return 1 }
-  if not shq(`{cc()} -O2 {ll} runtime/native/cocoa.ll -framework Cocoa -Wl,-needed_framework,GameController -Wl,-rpath,@loader_path -o {outbin}`) { return 1 }
+  # audio.ll (#22) is always linked here for the dev runner — unused snd_* are
+  # dead-stripped; the canonical `ludicc -o` path links it only when Audio.* is used.
+  if not shq(`{cc()} -O2 {ll} runtime/native/cocoa.ll runtime/native/audio.ll -framework Cocoa -Wl,-needed_framework,GameController -Wl,-needed_framework,AVFoundation -Wl,-rpath,@loader_path -o {outbin}`) { return 1 }
   if not save { run(`rm -f {ll}`) }
   print(`done. run: ./{outbin}   (from the repo root, so assets/ resolves)`)
   return 0