Validate Phase 4 lease-extend on a real busy day (~15+ spawn day)

First real enforcement of the rules-need-a-stress-test rule. Phase 4 (bin/lease-extend.sh, spawn-guard.sh::claim_spawn integration) shipped 2026-06-13 05:31 UTC. Mechanical verification at ship-time (4/4 tests green). Busy-day validation pending. Acceptance: on a day with ≥15 sub-agent spawns (likely Tuesday based on weekly cadence), measure trailing-24h fleet-task failed events. Accept Phase 4 as stress-tested if: - Zero unique tasks lease-expire during sub-agent activity (current Phase 3 bar is ~9 unique tasks on a 20-spawn day) - Zero poison-pill terminal events from lease-expiry on tasks with active subagent_spawned events If acceptance fails, file Phase 5 candidates: server-side hook in fleet-tasks (canonical fix per substrate-investigations/lease-renew-active-ciwatch-gap.md), or increase lease TTL to 30 min. This task remains queued until a busy-spawn day happens. Heartbeat that detects ≥15 spawns in a trailing-24h window should mark this in_progress and run the verification. **Until validated, the substrate-noise-scan streak is held at 0 + validation pending — not ticked.**