AssetResolver

Hook for adopting buffers from an external asset system (Pixi assetpack, IndexedDB cache, custom manifest) instead of (or in addition to) zvuk's URL fetcher. Returning `undefined` / `null` falls through to the URL fetch, so resolvers can mix cached and uncached sounds without branching at the call site. The resolver runs before any fetch — if it returns a buffer or URL, the URL list passed to `loadSound` is only used as the resolution key.