White-box blob round-trip probe

Public, no auth. Sibling of the KV probe: times a PUT then HEAD then DELETE from inside a serverless function, on the actual write path uploads use, so it catches write failure, disk-full, auth drift and tunnel degradation that a plain liveness check cannot. Reports the round trip under the same kv_ms field name so one canary parses both probes identically. One fixed path, written then deleted each probe.