fix: 支持跨版本隧道链路 (v6入v4出 / v4入v6出)

问题：selectTunnelDialHost 只检查同版本兼容 (v4->v4, v6->v6)，
导致 v6-only 入口节点连接 v4-only 出口节点时报错：
"节点链路不兼容"

修复：在 default 分支增加跨版本支持：
- fromV6 && toV4 → 返回出口 v4 地址
- fromV4 && toV6 → 返回出口 v6 地址

更新测试用例以反映新行为

go-backend/internal/http/handler/dual_stack_test.go

@@ -345,21 +345,39 @@ func TestSelectTunnelDialHost_V6Only_PreferV4Fallback(t *testing.T) {
 	}
 }
 
-func TestSelectTunnelDialHost_Incompatible(t *testing.T) {
+func TestSelectTunnelDialHost_CrossVersion_V4ToV6(t *testing.T) {
+	// v4-only -> v6-only: 跨版本支持，应成功返回 v6 地址
 	from := v4OnlyNode("from", "10.0.0.1")
 	to := v6OnlyNode("to", "2001:db8::2")
-	_, err := selectTunnelDialHost(from, to, "", "")
-	if err == nil {
-		t.Fatal("expected error for incompatible nodes (v4-only -> v6-only)")
+	host, err := selectTunnelDialHost(from, to, "", "")
+	if err != nil {
+		t.Fatalf("unexpected error for cross-version (v4-only -> v6-only): %v", err)
+	}
+	if host != "2001:db8::2" {
+		t.Fatalf("expected v6 address for cross-version, got %q", host)
 	}
 }
 
-func TestSelectTunnelDialHost_Incompatible_Reverse(t *testing.T) {
+func TestSelectTunnelDialHost_CrossVersion_V6ToV4(t *testing.T) {
+	// v6-only -> v4-only: 跨版本支持，应成功返回 v4 地址
 	from := v6OnlyNode("from", "2001:db8::1")
 	to := v4OnlyNode("to", "10.0.0.2")
+	host, err := selectTunnelDialHost(from, to, "", "")
+	if err != nil {
+		t.Fatalf("unexpected error for cross-version (v6-only -> v4-only): %v", err)
+	}
+	if host != "10.0.0.2" {
+		t.Fatalf("expected v4 address for cross-version, got %q", host)
+	}
+}
+
+func TestSelectTunnelDialHost_TrulyIncompatible(t *testing.T) {
+	// 真正不兼容：两个节点都没有任何 IP
+	from := &nodeRecord{Name: "empty-from", ServerIPv4: "", ServerIPv6: "", ServerIP: ""}
+	to := &nodeRecord{Name: "empty-to", ServerIPv4: "", ServerIPv6: "", ServerIP: ""}
 	_, err := selectTunnelDialHost(from, to, "", "")
 	if err == nil {
-		t.Fatal("expected error for incompatible nodes (v6-only -> v4-only)")
+		t.Fatal("expected error for nodes with no IP addresses")
 	}
 }
 

go-backend/internal/http/handler/mutations.go

@@ -3464,6 +3464,7 @@ func selectTunnelDialHost(fromNode, toNode *nodeRecord, ipPreference string, con
 			}
 		}
 	default:
+		// 同版本优先
 		if fromV4 && toV4 {
 			if host := pickNodeAddressV4(toNode); host != "" {
 				return host, nil
@@ -3474,6 +3475,17 @@ func selectTunnelDialHost(fromNode, toNode *nodeRecord, ipPreference string, con
 				return host, nil
 			}
 		}
+		// 跨版本支持：v6入v4出 / v4入v6出
+		if fromV6 && toV4 {
+			if host := pickNodeAddressV4(toNode); host != "" {
+				return host, nil
+			}
+		}
+		if fromV4 && toV6 {
+			if host := pickNodeAddressV6(toNode); host != "" {
+				return host, nil
+			}
+		}
 	}
 	return "", fmt.Errorf("节点链路不兼容：%s(v4=%t,v6=%t) -> %s(v4=%t,v6=%t)", nodeDisplayName(fromNode), fromV4, fromV6, nodeDisplayName(toNode), toV4, toV6)
 }