zoom to CRDT clock

[jchris]

we should have an official api for zooming to a clock point.

some considerations -- writes correspond to a single head, but reads to a multi head. you should be able to zoom to single or multi head.

defaults and documentation should make it clear that single head corresponds to a snapshot at time of write, but multi head introduces potential merges from non-causal actors.

[jchris]

now that #21 is closed we can read from _changes for the corresponding write clock crdt

[jchris]

loading from a snapshot as a hack https://github.com/fireproof-storage/dashboard/blob/main/src/lib/db.ts#L30