Fix std engine OnConnect / OnDisconnect callbacks

[FumingPower3925]

Problem

Config.OnConnect and Config.OnDisconnect are called by epoll and io_uring engines but silently ignored by the std engine. The connStateHook only tracks activeConns.

Unblocks

• Connection Hooks middleware (fix: arm64 io_uring detection slowdown and H2 body throughput drop #39) on std engine

Change

// engine/std/engine.go — connStateHook:
func (e *Engine) connStateHook(conn net.Conn, state http.ConnState) {
    switch state {
    case http.StateNew:
        e.metrics.activeConns.Add(1)
        if e.cfg.OnConnect != nil {
            e.cfg.OnConnect(conn.RemoteAddr().String())
        }
    case http.StateClosed, http.StateHijacked:
        e.metrics.activeConns.Add(-1)
        if e.cfg.OnDisconnect != nil {
            e.cfg.OnDisconnect(conn.RemoteAddr().String())
        }
    }
}

Performance

Two nil checks in connection-lifecycle code (not per-request). Branch prediction eliminates overhead when callbacks are nil.

Files

• engine/std/engine.go