Required, unlike the contiguous arm's nullable key. A contiguous receiver can recompute a missing key with one linear pass over bytes it already holds; a blocked one would have to inflate every block to do it. Making it non-nullable here forces the compression swap to compute and carry the key eagerly, while the source is still contiguous.
Structured-clone-safe description of a source, for handing it to a worker WITHOUT materialising it on the sending side.
blockedis declared here but not yet produced; it is the shape the compressed implementation will post.