task_snapshots gains via and read_at, an integer-bucket CHECK with a
tombstone exception, the read-ordering rule in task_external_changes and
the tasks_open view. task.missing names its task and a reason. Prototype
rerun on Node 24.21.0 and 26.8.1; a mutant without the read_at rule
reports the broker's own move as external. Lead decision 52 (B3).
Co-Authored-By: Claude Opus 5.5 <[email protected]>