Signal session readiness only after durable child writes

A reusable ingestion rule from the Codex Session Logging → Sesh integration: never let a parent session timestamp double as readiness if it must be written before FK-backed messages/events. Concurrent first writes can reset or prematurely advance that timestamp. The safe pattern is an insert-if-absent parent with a non-discoverable sentinel, followed by the durable idempotent child write, metadata update that omits the watermark, and a service-role-only database-atomic monotonic watermark advance. This keeps retries recoverable and prevents a correct session with missing evidence from becoming temporarily or permanently indexed as ready.