Second review round on field-merge sending:
- A whole successor or a delete after a merge keeps the revision it was
  made from; a successor behind recovered work the canvas never drew keeps
  its own fields.
- The canvas judges liveness as it hands work over, before the metadata
  sync it waits behind; an explicit null means made offline.
- Retries (Keep mine, restored pages) never inherit live status.
- Revision-checked deletes claim the claimable revision, as whole patches
  do; an acked add keeps its base while its overlay waits for the read.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>