Third review round on field-merge sending:
- A successor names its change from the predecessor's payload when it is
  made, while it is known whether the canvas drew the predecessor, and is
  saved that way, so a change set back in flight survives a restart.
  Promotion only keeps a whole write or delete on the revision it was made
  from.
- A send that failed drops its live status, so its retry is checked.

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