sync: a delta's signatures as a blob of their own

A sync message larger than the threshold for its peer (16 chunks, lower
on a slow link or after a message to it failed whole) goes as a delta:
the peer fetches its changes in chunks, from any node holding them. Its
signatures, each writer's latest with changes in it, went in the
envelope offering it: up to 65536 of them, 6 MiB, in one message, the
size the delta exists to avoid. Nor did an Automerge message count its
signatures (one per writer in a whole document pulled) towards the
threshold.

A delta's signatures are now a blob of their own: the offerer packs and
keeps them (Blobs::keep, the latest 64), and the spec names them by
BLAKE3 hash and size (DeltaSpec::signatures, None where no signature
vouches for its changes). They are served as content (BlobId::Content),
so their hash checks them. The receiver fetches them from the offerer
alone, as another node holding the changes holds signatures of its own,
before the changes, and offers them under the write lock just before
judging the changes, so they wait for them as before. The envelope of a
delta carries none. An Automerge message's signatures count towards its
size: past the threshold, it goes as a delta.

Wire version 49.

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