maintainer-registry: registry_writer attributes writes to the wrong agent

Found 2026-09-05 by hitting it, not by reasoning about it: bin-venus-cc and pm-llmmsgsrv-cc wrote row 158 concurrently. Nothing was lost only because the columns were disjoint (name/lastVerified/historyNote vs the backup block). The defect is in the guard: registry_writer holds exactly one row and records who DECLARED within a 12h TTL, not who PERFORMED a write. So pm-llmmsgsrv-cc's write was attributed to bin-venus-cc (declared 2026-09-05 08:13:45) - the table that exists specifically to name the writer names the wrong agent whenever two agents work inside one declaration window, silently, with no error and nothing to notice it by. A 12h TTL makes overlap the normal case, not the edge. Wanted shape (bin-venus-cc and pm-llmmsgsrv-cc agree): stamp the writer PER ROW-CHANGE, not per window - a change-log row or per-row writer/changedAt columns - so attribution cannot be inherited from a stale declaration. Registry: /var/lib/llmmsg-srv/maintainer-agents.sqlite. Owners: bin-venus-cc primary, pm-llmmsgsrv-cc secondary. Related history: the 2026-08-21 no-WHERE UPDATE that overwrote the backup block on all 152 rows is the same single-writer-assumption failure one level up.