RedWire frame handlers — pure request/response transformations
shared by the RedWire session loop. Each handler takes a parsed
payload and returns a length-prefixed response envelope; the
session adapts the envelope into a RedWire frame via
rewrap_handler_response.
Discriminator byte every RedWire client sends as the very first
byte off the wire. The service-router detector keys off this
(and so does the standalone listener path).
Start a RedWire listener on an already-bound TCP listener (used
by the service router which owns the public socket and proxies
to a loopback redwire backend).