Refactor internal/proxy/rtmp.go to expose functional-option seams
(listen, newConnection, newHandshake, newProtocol, newBackend, dial)
and widen the proxy server and connection to net.Listener / net.Conn
so fakes can be injected. Tighten the identify switch in serve() to
a real switch on CommandName.
Add internal/proxy/rtmp_test.go covering rtmpProxyServer (constructor
defaults, options, Close, listen error, endpoint normalization,
accept-loop, graceful shutdown), rtmpConnection (defaults, serve
handshake/protocol error paths, identify-loop branches, newBackend
invocation contract), and rtmpClientToBackend (Close, Connect happy
and error paths, publish, play). rtmp.go statement coverage rises to
76.9% with every function exercised.
