refactor(handler/forward/local): split handler into 6 focused files with 40 tests at 100% coverage

Extract Handle into handleRawForwarding, handleSniffedProtocol, SnifferBuilder,
newRecorderObject, and checkRateLimit across forward.go, sniffing.go, and
util.go. Replace redundant x509.ParseCertificate with tlsCert.Leaf (already
populated by tls.LoadX509KeyPair since Go 1.23).

handler/forward/local/util.go

@@ -0,0 +1,50 @@
+package local
+
+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
+}