Interrupt Founding

The founder's ✕ STOP (reconciliation-2 spec §6), mirroring the sessions interrupt: stamp interrupt_requested_at and let the segment loop wrap up at its next boundary — between tool calls, so the transcript stays paired.

Two 409s, both "nothing to stop": no live claim per the same liveness predicate segment_running uses, and a conditional UPDATE that matched zero rows because the segment finished between that read and the write. The stamp is idempotent — a second press returns the first's timestamp — and the founder's own next turn clears it.