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/audio.ts

@@ -0,0 +1,76 @@
+// Lightweight game audio: one looping "scene music" track plus synthesized one-shot
+// SFX. Zero dependencies. Browsers block autoplay until a user gesture, so music/SFX
+// are primed on the first pointer interaction.
+type Sfx = 'advance' | 'choice' | 'sting'
+const MUSIC_VOLUME = 0.5
+
+let ctx: AudioContext | null = null
+function audioContext() {
+  if (!ctx) { const AC = window.AudioContext || (window as unknown as { webkitAudioContext?: typeof AudioContext }).webkitAudioContext; if (AC) ctx = new AC() }
+  return ctx
+}
+
+const music = typeof Audio !== 'undefined' ? new Audio() : null
+if (music) music.loop = true
+let currentUrl: string | null = null
+let muted = false
+let pending = false
+let fadeTimer: number | undefined
+
+function fade(target: number, done?: () => void) {
+  if (!music) return
+  window.clearInterval(fadeTimer)
+  const step = (target - music.volume) / 12 || (target > music.volume ? 0.1 : -0.1)
+  fadeTimer = window.setInterval(() => {
+    if (!music) return
+    const next = music.volume + step
+    if ((step >= 0 && next >= target) || (step <= 0 && next <= target)) { music.volume = target; window.clearInterval(fadeTimer); done?.() }
+    else music.volume = Math.max(0, Math.min(1, next))
+  }, 40)
+}
+
+function startMusic() {
+  if (!music || !currentUrl || muted) return
+  const promise = music.play()
+  if (promise) promise.then(() => { pending = false; fade(MUSIC_VOLUME) }).catch(() => { pending = true })
+}
+
+export const audio = {
+  // A null url inherits whatever is already playing; a new url crossfades to it.
+  setMusic(url: string | null) {
+    if (!music || url === currentUrl) return
+    currentUrl = url
+    if (!url) { fade(0, () => music.pause()); pending = false; return }
+    music.src = url
+    music.volume = 0
+    startMusic()
+  },
+  toggleMute() {
+    muted = !muted
+    if (muted) fade(0)
+    else if (currentUrl) startMusic()
+    return muted
+  },
+  isMuted() { return muted },
+  sfx(kind: Sfx) {
+    if (muted) return
+    const context = audioContext()
+    if (!context || context.state !== 'running') return
+    const osc = context.createOscillator(), gain = context.createGain()
+    osc.type = 'sine'
+    osc.frequency.value = kind === 'choice' ? 540 : kind === 'sting' ? 300 : 400
+    const now = context.currentTime
+    gain.gain.setValueAtTime(0.0001, now)
+    gain.gain.exponentialRampToValueAtTime(0.07, now + 0.008)
+    gain.gain.exponentialRampToValueAtTime(0.0001, now + 0.12)
+    osc.connect(gain).connect(context.destination)
+    osc.start(now); osc.stop(now + 0.13)
+  },
+  // Resume the context and retry pending music on a user gesture.
+  resume() {
+    void audioContext()?.resume?.()
+    if (pending) startMusic()
+  },
+}
+
+if (typeof window !== 'undefined') window.addEventListener('pointerdown', () => audio.resume())