Stable builder for embedding the proxy with custom extension points. Obtain it
via Proxy::builder. Every seam defaults to the built-in behaviour, so a
builder with no overrides is identical to Proxy::bind. The enterprise plugs
a distributed rate-limit store or premium modules here without forking
(BOUNDARY §4).
Handle that can hot-reload a running proxy’s configuration. Obtained via
Proxy::reloader(); cheap to clone (an Arc). Used by the SIGHUP task in the
binary and directly by tests.