M8 Scheduling Evidence

Coverage: five-field cron validation, IANA timezones, UTC nominal runs, durable schedule CRUD, live next-run updates, misfire handling, and no-overlap delivery through the execution enqueue guard.
Full verification: make check passed with 81 unit/contract, 40 integration, 10 fault, and 17 security tests, plus Ruff, mypy, frontend typecheck, and frontend production build.
Scope: retention and garbage collection are M9 work.