refactor(handler/forward/remote): split handler into 6 focused files with 41 tests

Split the 339-line handler monolith and 1169-line test blob into 11 files
(5 source + 6 test) following the handler/forward/local/ pattern.

Source files: handler.go (core), forward.go (raw forwarding),
sniffing.go (SnifferBuilder + protocol dispatch), util.go (errors +
recorder + rate limit), metadata.go (config parsing).

Fixes: add sync.Mutex for hop access (race), use sentinel errors for
errors.Is compatibility, add nil-addr guard in checkRateLimit, reuse
SnifferBuilder via Build() instead of per-call construction.

handler/forward/remote/util.go

@@ -0,0 +1,50 @@
+package remote
+
+import (
+	"context"
+	"errors"
+	"net"
+	"time"
+
+	xctx "github.com/go-gost/x/ctx"
+	xrecorder "github.com/go-gost/x/recorder"
+)
+
+var (
+	errRouterNotAvailable = errors.New("router not available")
+	errNodeNotAvailable   = errors.New("node not available")
+)
+
+// newRecorderObject creates a HandlerRecorderObject populated with connection
+// metadata (service, addresses, network type, session ID, client address).
+func (h *forwardHandler) newRecorderObject(ctx context.Context, conn net.Conn, start time.Time) *xrecorder.HandlerRecorderObject {
+	ro := &xrecorder.HandlerRecorderObject{
+		Service:    h.options.Service,
+		RemoteAddr: conn.RemoteAddr().String(),
+		LocalAddr:  conn.LocalAddr().String(),
+		Network:    "tcp",
+		Time:       start,
+		SID:        xctx.SidFromContext(ctx).String(),
+	}
+	if srcAddr := xctx.SrcAddrFromContext(ctx); srcAddr != nil {
+		ro.ClientAddr = srcAddr.String()
+	}
+	if _, ok := conn.(net.PacketConn); ok {
+		ro.Network = "udp"
+	}
+	return ro
+}
+
+func (h *forwardHandler) checkRateLimit(addr net.Addr) bool {
+	if h.options.RateLimiter == nil {
+		return true
+	}
+	if addr == nil {
+		return true
+	}
+	host, _, _ := net.SplitHostPort(addr.String())
+	if limiter := h.options.RateLimiter.Limiter(host); limiter != nil {
+		return limiter.Allow(1)
+	}
+	return true
+}