Rates how severe a finding or bug is if it is real — the blast-radius axis, complementary to confidence's is-it-real axis. Emits a lowercase tier (critical / high / medium / low) from a fixed axis rubric, then applies one deterministic, executable path floor (auth / billing / migration / infra / secrets paths) plus heuristic escalators (data-loss, security, concurrency shapes) — both scoped to reachable production code, never test / fixture / generated paths. Callers own how they gate on the tier and map it to their own blocking rules; the skill stays policy-free. Use to triage a review finding or a bug. Triggers on "how severe", "rate severity", "severity check", "triage this finding", "how bad is this", "/severity".