middleware/websocket: Conn.Param always returns "" (route params are never captured), and on epoll and io_uring Conn.IP and Conn.RemoteAddr return "" and nil

[FumingPower3925]

Summary

In middleware/websocket:

• Conn.Param always returns "", on every engine, because the route params are never captured.
• On epoll and io_uring, Conn.IP() returns "" and Conn.RemoteAddr() returns nil.

#714 guessed that Conn.Param "is very likely affected" by the query aliasing. The audit it called for found that the real defect is different: Param is never captured at all. Neither defect is a view retention.

Mechanism

• Param. Conn.Param reads c.params (middleware/websocket/conn.go:950 at 9f4d89b). Nothing in the package assigns params: setupConn (websocket.go:308-322) sets only headers and query, and git log -S'ws.params' finds no commit that ever did. This has been the case since the middleware landed in v1.3.4 (feat: v1.3.4 realtime middleware (WebSocket + SSE + Protobuf) #229).
• IP and RemoteAddr. Both read c.conn (conn.go:880-925). Only the hijack path sets it (newConn, used by the std engine). The engine path builds its Conn with newEngineConn (conn.go:135-157), which has no net.Conn, and the peer address that c.RemoteAddr() holds on the Context (context_request.go:654) is not captured.

Reproduction

The evidence test is TestC714EvidenceParamAndIP, kept with the #714 evidence. The route is GET /ws/:room, and the handler reports c.Param("room"), c.IP() and c.RemoteAddr() for an upgrade to /ws/lobby.

engine	Param("room")	IP()	RemoteAddr()
std	""	127.0.0.1	127.0.0.1:<port>
epoll	""	""	<nil>
io_uring	""	""	<nil>

Docker linux/arm64 (--cpus 4), kernel 7.0.12-linuxkit, memlock 8 MiB, go test -race.

Fix direction

Capture the route params and the peer address at upgrade, next to captureQuery. Clone every string, for the reason #714 gives: on the native engines a param value is a view of the receive buffer, and the Conn outlives the request. Keep the peer address as a string for IP(), and return a net.Addr built from it for RemoteAddr(), so the engine path answers what the hijack path does.