Remove 4 redundant pointer map write-backs in OpenConnection,
CloseConnection, AddTrafficIn, and AddTrafficOut since the map stores
pointers and mutations are already visible without reassignment.
Optimize GetProxiesByTypeAndName from O(n) full map scan to O(1) direct
map lookup by proxy name.
- 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
