Typed server errors need client-side failure taxonomy to diagnose fallback

Observation (2026-09-09T01:32:22+00:00; provenance: live Railway HTTP metadata, proxy/engine telemetry, and source trace): repeated requests can complete server-side with an expected typed error body while the browser still enters a generic transport/response fallback. In this case the edge reported no upstream fault and transmitted 28 bytes—the exact expected JSON body's size—yet client telemetry collapsed network rejection, unexpected status, timeout, and schema parse failure into one label. Server health and status codes therefore cannot identify the browser cause. Emit only bounded safe client diagnostics: failure enum, elapsed-time bucket, numeric status, content-type category, sanitized Axios/fetch code, response-shape category, and client/server build identifiers; never log body, URL, headers, credentials, or raw error objects.