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/test.ludic

@@ -256,6 +256,7 @@ function cmd_test() -> int {
   # deterministic input record/replay — fed one key per poll from stdin.
   feat_case("library/input_actions", "  xwa", "1 0 1 1 0 1 0", "input_actions.ludic (#7 action maps + rebinding + deterministic replay)")
   feat_case("library/input_device", "", "1 1 0 1 0 1 71 -71 5 1 3 1 2 1 0 0 1", "input_device.ludic (#50 multi-key held state + analog axis/vector + mouse + gamepad/touch + full-state replay)")
+  feat_case("library/audio", "", "0 0 0 0 1", "audio.ludic (#22 Audio.* load/play/music/volume/pitch/stop/is_playing — headless no-op)")
 
   feat_case("events/recurse", "", "16", "recurse.ludic (EV6: re-entrant emit is depth-bounded, no runaway cycle)")
   net_case("events/scoped", "2")