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>

simulator/clips.py

@@ -82,7 +82,7 @@ def _scale_from_dict(s: dict[str, Any]) -> Scale:
     """
     pool = tuple(s.get("pool", []))
     clip_id = s.get("clip_id") or (pool[0] if pool else "")
-    return Scale(id=s["id"], clip_id=clip_id, pool=pool)
+    return Scale(id=s["id"], clip_id=clip_id, pool=pool, audio=s.get("audio", ""))
 
 
 def load_ring(path: str | Path) -> ScaleRing | None:
@@ -119,6 +119,7 @@ def ring_to_dict(ring: ScaleRing, clips: list[Clip]) -> dict:
                 "id": s.id,
                 "clip_id": s.clip_id,
                 "title": titles.get(s.clip_id, s.id),
+                "audio": s.audio,
                 "pool": [
                     {"clip_id": cid, "title": titles.get(cid, cid)}
                     for cid in s.members