feat(audio): Scale.audio field + per-scale soundtrack wiring through the manifest

Scale gains audio:str='' (back-compat). clips.py reads/serializes it; ring_to_dict
exposes it; build_pool_manifest emits SCALE_AUDIO per scale. Manifest regenerated.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

player/ring.py

@@ -63,6 +63,7 @@ class Scale:
     id: str
     clip_id: str
     pool: tuple[str, ...] = ()
+    audio: str = ""              # per-scale soundtrack, relative to /media/audio/ (audio spec §5.1)
 
     @property
     def members(self) -> tuple[str, ...]: