White-box KV round-trip probe

Public, no auth. Times a SET+GET from INSIDE a serverless function, so it measures the exact path renders and crons use - a probe run next to the KV host could not see the degradation this exists to catch. Abuse-safe: one fixed key with a short TTL, so hammering it overwrites one key rather than amplifying writes. It answers 200 WITH THE MEASUREMENT whenever KV responds at all and puts the verdict in degraded; 503 means KV threw.