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_test.go

@@ -0,0 +1,145 @@
+package remote
+
+import (
+	"context"
+	"net"
+	"testing"
+	"time"
+
+	"github.com/go-gost/core/limiter/rate"
+	xctx "github.com/go-gost/x/ctx"
+)
+
+// ---------------------------------------------------------------------------
+// newRecorderObject
+// ---------------------------------------------------------------------------
+
+func TestNewRecorderObject_TCP(t *testing.T) {
+	h := newInitdHandler()
+	conn := newStringConn(nil)
+	start := time.Now()
+
+	ro := h.newRecorderObject(context.Background(), conn, start)
+
+	if ro.Network != "tcp" {
+		t.Errorf("expected network tcp, got %s", ro.Network)
+	}
+	if ro.RemoteAddr != "10.0.0.1:12345" {
+		t.Errorf("expected remote 10.0.0.1:12345, got %s", ro.RemoteAddr)
+	}
+	if ro.LocalAddr != "127.0.0.1:8080" {
+		t.Errorf("expected local 127.0.0.1:8080, got %s", ro.LocalAddr)
+	}
+	if !ro.Time.Equal(start) {
+		t.Errorf("expected time %v, got %v", start, ro.Time)
+	}
+}
+
+func TestNewRecorderObject_SID(t *testing.T) {
+	h := newInitdHandler()
+	conn := newStringConn(nil)
+	ctx := xctx.ContextWithSid(context.Background(), xctx.Sid("remote-sid"))
+
+	ro := h.newRecorderObject(ctx, conn, time.Now())
+
+	if ro.SID != "remote-sid" {
+		t.Errorf("expected SID 'remote-sid', got '%s'", ro.SID)
+	}
+}
+
+func TestNewRecorderObject_NilSrcAddr(t *testing.T) {
+	h := newInitdHandler()
+	conn := newStringConn(nil)
+
+	ro := h.newRecorderObject(context.Background(), conn, time.Now())
+
+	if ro.ClientAddr != "" {
+		t.Errorf("expected empty ClientAddr, got %s", ro.ClientAddr)
+	}
+}
+
+func TestNewRecorderObject_SrcAddr(t *testing.T) {
+	h := newInitdHandler()
+	conn := newStringConn(nil)
+	srcAddr := &net.TCPAddr{IP: net.IPv4(192, 168, 1, 1), Port: 54321}
+	ctx := xctx.ContextWithSrcAddr(context.Background(), srcAddr)
+
+	ro := h.newRecorderObject(ctx, conn, time.Now())
+
+	if ro.ClientAddr != "192.168.1.1:54321" {
+		t.Errorf("expected ClientAddr '192.168.1.1:54321', got '%s'", ro.ClientAddr)
+	}
+}
+
+func TestNewRecorderObject_UDP(t *testing.T) {
+	h := newInitdHandler()
+	conn := &packetConn{newStringConn(nil)}
+
+	ro := h.newRecorderObject(context.Background(), conn, time.Now())
+
+	if ro.Network != "udp" {
+		t.Errorf("expected network udp, got %s", ro.Network)
+	}
+}
+
+// ---------------------------------------------------------------------------
+// checkRateLimit
+// ---------------------------------------------------------------------------
+
+func TestCheckRateLimit_NilLimiter(t *testing.T) {
+	h := newInitdHandler()
+	conn := newStringConn(nil)
+
+	if !h.checkRateLimit(conn.RemoteAddr()) {
+		t.Error("expected true when RateLimiter is nil")
+	}
+}
+
+func TestCheckRateLimit_NilAddr(t *testing.T) {
+	h := newInitdHandler(withRateLimiter(&stubRateLimiter{
+		limiterFn: func(key string) rate.Limiter {
+			return &stubLimiter{allowFn: func(n int) bool { return false }}
+		},
+	}))
+
+	if !h.checkRateLimit(nil) {
+		t.Error("expected true when addr is nil")
+	}
+}
+
+func TestCheckRateLimit_Allowed(t *testing.T) {
+	h := newInitdHandler(withRateLimiter(&stubRateLimiter{
+		limiterFn: func(key string) rate.Limiter {
+			return &stubLimiter{allowFn: func(n int) bool { return true }}
+		},
+	}))
+	conn := newStringConn(nil)
+
+	if !h.checkRateLimit(conn.RemoteAddr()) {
+		t.Error("expected true when limiter allows")
+	}
+}
+
+func TestCheckRateLimit_Blocked(t *testing.T) {
+	h := newInitdHandler(withRateLimiter(&stubRateLimiter{
+		limiterFn: func(key string) rate.Limiter {
+			return &stubLimiter{allowFn: func(n int) bool { return false }}
+		},
+	}))
+	conn := newStringConn(nil)
+
+	if h.checkRateLimit(conn.RemoteAddr()) {
+		t.Error("expected false when limiter blocks")
+	}
+}
+
+func TestCheckRateLimit_NoLimiterForKey(t *testing.T) {
+	h := newInitdHandler(withRateLimiter(&stubRateLimiter{
+		limiterFn: func(key string) rate.Limiter { return nil },
+	}))
+	conn := newStringConn(nil)
+
+	if !h.checkRateLimit(conn.RemoteAddr()) {
+		t.Error("expected true when no limiter found for key")
+	}
+}