Add per-node scene music volume control

Author a music track and volume (0-100%) per story node; the runtime
scales it to 0-1 and applies it without restarting the track when only
the level changes. Adds migration 024, threads music_volume through the
repository/runtime, and gives the node inspector a volume slider.

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

src/audio.ts

@@ -2,7 +2,7 @@
 // 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 baseVolume = 0.5 // authored scene volume (0-1); mute overrides to 0
 
 let ctx: AudioContext | null = null
 function audioContext() {
@@ -43,13 +43,16 @@ function fade(target: number, done?: () => void) {
 function startMusic() {
   if (!music || !currentUrl || muted) return
   const promise = music.play()
-  if (promise) promise.then(() => { pending = false; fade(MUSIC_VOLUME) }).catch(() => { pending = true })
+  if (promise) promise.then(() => { pending = false; fade(baseVolume) }).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
+  // The same url with a new volume just adjusts the level (no restart).
+  setMusic(url: string | null, volume?: number) {
+    if (!music) return
+    if (volume !== undefined) baseVolume = Math.max(0, Math.min(1, volume))
+    if (url === currentUrl) { if (volume !== undefined && currentUrl && !muted) fade(baseVolume); return }
     currentUrl = url
     if (!url) { fade(0, () => music.pause()); pending = false; return }
     music.src = url
@@ -59,7 +62,7 @@ export const audio = {
   toggleMute() {
     muted = !muted
     if (muted) fade(0)
-    else if (currentUrl) startMusic()
+    else if (currentUrl) { if (music && music.paused) startMusic(); else fade(baseVolume) }
     return muted
   },
   isMuted() { return muted },