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/narrative.tsx

@@ -2,7 +2,7 @@ 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; musicUrl?: 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; musicVolume?: number; 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 }