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/audio-is_playing.md

@@ -0,0 +1,23 @@
+---
+id: audio-is_playing
+name: Audio.is_playing
+category: audio
+kind: namespace-method
+tokens: Audio.is_playing
+sig: Audio.is_playing(id) -> bool
+tip: Is this sound playing?
+order: 9
+ns: Audio
+member: is_playing
+---
+
+Returns whether the sound <code>id</code> is currently playing. Always <code>false</code> in a headless build.
+
+```ludic
+program Demo {
+  entry {
+    let sfx = Audio.load("blip.wav")
+    if Audio.is_playing(sfx) { print(1) }
+  }
+}
+```