Extract two shared helpers to eliminate duplicated code across STCP,
SUDP, and XTCP visitors:
- dialRawVisitorConn: handles ConnectServer + NewVisitorConn handshake
(auth, sign key, 10s read deadline, error check)
- wrapVisitorConn: handles encryption + pooled compression wrapping,
returning a recycleFn for pool resource cleanup
SUDP is upgraded from WithCompression to WithCompressionFromPool,
aligning with the pooled compression used by STCP and XTCP.
- Replace duplicate parseBasicAuth with existing httppkg.ParseBasicAuth
- Extract buildDomains helper in BaseProxy for HTTP/HTTPS/TCPMux proxies
- Extract toProxyStats helper to deduplicate ProxyStats construction
- Extract startVisitorListener helper in BaseProxy for STCP/SUDP proxies
- Extract acceptLoop helper in BaseVisitor for STCP/XTCP visitors
