undo/redo support via zoom to CRDT clock

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.

The issue names no files, tests, or entry points. Start by tracing the existing CRDT clock and read/write APIs, then establish how single- and multi-head zooming should behave. Done means an official API, documented defaults, and coverage for snapshot versus non-causal merge behavior.

Written by the indexing model from the issue text.