126 only guards writes to compatibility; a later transceivers.speed_gbps update
(e.g. daily enrich-modulation-fec.sh self-heal) never re-validated existing
compatible spec_match rows. Found live 2026-07-12: 189 rows had drifted
physically-impossible again. One-time remediation (189 rows -> incompatible,
backed up to compat_speed_drift_backup_20260712) + AFTER UPDATE OF speed_gbps
trigger on transceivers closes the gap for good. check_switch_quality() passes.
