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>

src/narrative.tsx

@@ -1,7 +1,8 @@
 import { useEffect, useMemo, useRef, useState, type FC } from 'react'
+import { audio } from './audio'
 
 export type RuntimeUtterance = { id: string; utterer: 'npc' | 'player'; speaker: { name: string; role: string }; poseUrl: string | null; text: string; childIds: string[]; terminalKey: string | null }
-export type RuntimeNode = { id: string; kind: 'cutscene' | 'dialogue' | 'level'; label: string; componentKey?: string | null; levelSlug?: string | null; utterances?: RuntimeUtterance[]; rootId?: string | null }
+export type RuntimeNode = { id: string; kind: 'cutscene' | 'dialogue' | 'level'; label: string; componentKey?: string | null; levelSlug?: string | null; musicUrl?: string | null; utterances?: RuntimeUtterance[]; rootId?: string | null }
 export type PlaythroughSummary = { id: string; mysterySlug: string; levelSlug: string | null; status: string }
 export type PlaythroughState = { playthrough: PlaythroughSummary; node: RuntimeNode | null }
 
@@ -84,6 +85,7 @@ export function DialoguePlayer({ node, onExit, inline, startId }: { node: { utte
   }, [currentId, fullText, reduced]) // eslint-disable-line react-hooks/exhaustive-deps
 
   const pick = (choice: RuntimeUtterance) => {
+    if (!inline) audio.sfx('choice')
     if (choice.childIds.length > 0) setCurrentId(choice.childIds[0])
     else onExit(choice.terminalKey ?? undefined)
   }
@@ -93,6 +95,7 @@ export function DialoguePlayer({ node, onExit, inline, startId }: { node: { utte
     if (!done) { setCharCount(fullText.length); return }
     if (children.length === 0) { onExit(current.terminalKey ?? undefined); return }
     if (options.length > 0) return // a branch — wait for a choice
+    if (!inline) audio.sfx('advance')
     setCurrentId(children[0].id) // linear next line
   }
   useEffect(() => {