chore(assets): assets.json에 group/referencedAt 추가 및 AssetManager 참조 추적

lib/assets/assetManager.ts

@@ -38,6 +38,7 @@ export class AssetManager {
   private readonly allowed: Set<string>
   private byHash = new Map<string, AssetRef>()
   private byOriginal = new Map<string, string>() // originalName -> path
+  private referenced = new Set<string>() // originalName referenced via rewrite
 
   constructor(opts: AssetManagerOptions) {
     this.maxBytes = opts.maxBytes
@@ -91,8 +92,13 @@ export class AssetManager {
     if (urlOrLocalRef.startsWith('local:')) {
       const key = urlOrLocalRef.slice('local:'.length)
       const p = this.byOriginal.get(key)
+      if (p) this.referenced.add(key)
       return p ?? urlOrLocalRef
     }
     return urlOrLocalRef
   }
+
+  referencedList(): string[] {
+    return Array.from(this.referenced)
+  }
 }