Read-only execution-lineage timeline for one inbound record, chronological.

Sibling of MailglassAdmin.Operator.Timeline (the design contract). Rows are ExecutionRun projections (Pitfall 7 — the lineage schema, NOT the replay-run schema) from the internal inbound timeline gateway, each carrying source (:fresh/:replay), mailbox, outcome, outcome_reason, executed_at, and the run id.