Sound: per-node scene music + synthesized SFX (zero-dep)

- migration 023: story_nodes.music_asset_id (references the asset store).
- src/audio.ts: a tiny audio manager — one looping scene-music track (fade,
  dedup, gesture-primed autoplay) and synthesized one-shot SFX; global mute.
- Runtime: RuntimeNode.musicUrl; music follows the current node (null inherits,
  a finished playthrough stops). SFX blips on dialogue advance/choice (not in the
  editor preview). Floating mute toggle during play.
- Graph inspector: a "Scene music" picker sourced from uploaded audio assets.

No external library (we did not adopt react-winamp — it is a React 16 CRA app,
not an installable package). Bundle grew ~2 KB.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

migrations/023_story_node_music.sql

@@ -0,0 +1,4 @@
+-- Optional scene music per story node. The runtime plays/loops it while the node
+-- is active; a NULL value inherits whatever is already playing (so a track set on
+-- one node carries through the region until another node changes it).
+ALTER TABLE osint.story_nodes ADD COLUMN music_asset_id UUID REFERENCES osint.assets(id);