Function planBlobSweep

planBlobSweep(
    store: BlobStore,
    referenced: Set<string>,
    options?: SweepOptions & {
        metaProvider?: (hash: string) => Promise<BlobMeta | null>;
    },
): Promise<SweepDecision>
Plan a blob sweep against store, retaining everything in referenced.

The store's list() returns hashes only; size info comes from a metaProvider (default: walks store.get(hash).byteLength).
Parameters
- store: BlobStore
- referenced: Set<string>
- Optionaloptions: SweepOptions & { metaProvider?: (hash: string) => Promise<BlobMeta | null> }
  - OptionalmetaProvider?: (hash: string) => Promise<BlobMeta | null>
    
    Optional fast-path metadata lookup; default streams via store.get.
Returns Promise<SweepDecision>