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_call.ludic

@@ -216,6 +216,21 @@ function emit_ns_call(ns: pointer, meth: pointer, e: Node) -> Val {
     if (meth == "touch_y")      { bare = "input_touch_y";      push(labels, "index") }
     if (meth == "set_touch")    { bare = "input_set_touch";    push(labels, "index"); push(labels, "x"); push(labels, "y"); push(labels, "active") }
   }
+  # Audio.* (#22) — sfx/music playback over the platform audio backend
+  # (runtime/native/audio.ludic + audio.ll). Playback is out-of-band; the
+  # triggers are ordinary frame-driven calls, so a replay fires the same sounds.
+  if (ns == "Audio") {
+    if (meth == "load")       { bare = "audio_load";       push(labels, "path") }
+    if (meth == "play")       { bare = "audio_play";       push(labels, "id") }
+    if (meth == "play_sound") { bare = "audio_play";       push(labels, "id") }
+    if (meth == "play_music") { bare = "audio_play_music"; push(labels, "id") }
+    if (meth == "stop")       { bare = "audio_stop";       push(labels, "id") }
+    if (meth == "stop_music") { bare = "audio_stop_music" }
+    if (meth == "stop_all")   { bare = "audio_stop_all" }
+    if (meth == "volume")     { bare = "audio_volume";     push(labels, "v") }
+    if (meth == "pitch")      { bare = "audio_pitch";      push(labels, "v") }
+    if (meth == "is_playing") { bare = "audio_is_playing"; push(labels, "id") }
+  }
   # Phase 3: the bare reflection / networking / process builtins, namespaced.
   # Each is a pure alias — the callee is rewritten to the bare name below.
   if (ns == "World") {