Two gate findings on the worklog-event-timestamp feature (#5700/#5739):
1. Thinking-row timestamps disappeared after settlement. Reasoning source events
carry no server timestamp; the live DOM showed a fallback time but the settled
scene row rebuilt with created_at:null. Now stamp created_at on the source event
the first time it's seen (messages.js apply path) and carry row.created_at
through snapshot hydration, so the timestamp survives live->settled.
2. A garbage huge numeric timestamp (e.g. 1e20) passed the finite/>0 checks and
rendered literal 'Invalid Date'. _timestampSeconds now rejects epochs outside
JavaScript's valid Date range. Added a regression test.
Co-authored-by: rodboev. Gate findings from adversarial Codex re-gate.
