Canary check test coverage batch 1: Check 1 + Check 1c

See memory/2026-06-23.md P1 section for full spec. Add test scripts mocking the fleet-tasks API for Check 1 (sweep chain liveness) and Check 1c (sweep-scheduler cron heartbeat). Both must pass three cases: alert-on-fail, alert-on-stale, silent-on-healthy. Update notes/canary-fixture-coverage-policy.md audit table.