the call outcome now rides the frame as Result<Response, WireError>, so
a rejection reaches the caller with its message and the pooled
connection stays up. only a real transport failure closes it now; a
malformed request comes back as a rejection too.
the inter-service payload is already raw bytes; postcard sends it as
length-prefixed binary instead of base64 stuffed into a json envelope.
drops the base64 module and the per-field serde shims.