fix(link): read fm and the mKCP fields from vmess links (#6809)

#6480 taught the vless/trojan importers to rebuild the mKCP header and
seed and left the vmess importers ignoring fm. The vmess emitter writes
the finalmask as fm, the mKCP header as type and the seed as path, plus
mtu/tti. Both importers dropped all of them, so a vmess mKCP inbound
with a seed, a header or any finalmask UDP mask (salamander, ...) came
back from an outbound subscription, a client external link or the
outbound form's link import as an outbound that cannot connect.

Feed those fields through the same finalmask and kcp helpers the
url-param protocols use, in the Go and TS importers.

internal/util/link/outbound.go

@@ -205,8 +205,18 @@ func parseVmess(link string) (*ParseResult, error) {
 				},
 			}
 		}
+	case "kcp":
+		applyTransport(stream, url.Values{"mtu": {strconv.Itoa(num(j["mtu"]))}, "tti": {strconv.Itoa(num(j["tti"]))}})
 	}
 
+	// The vmess object carries fm as-is and the mKCP header and seed as type and path.
+	maskParams := url.Values{"fm": {getString(j, "fm", "")}}
+	if network == "kcp" {
+		maskParams.Set("headerType", getString(j, "type", ""))
+		maskParams.Set("seed", getString(j, "path", ""))
+	}
+	applyFinalMask(stream, maskParams)
+
 	if security == "tls" {
 		tls := stream["tlsSettings"].(map[string]any)
 		tls["serverName"] = getString(j, "sni", "")

internal/util/link/outbound_helpers_test.go

@@ -286,6 +286,11 @@ func mkcpLegacyMasks(t *testing.T, res *ParseResult) []mkcpMask {
 	return out
 }
 
+func vmessKcpLink(fields string) string {
+	inner := `{"v":"2","add":"h.com","port":443,"id":"11111111-2222-4333-8444-555555555555","net":"kcp","tls":"none",` + fields + `}`
+	return "vmess://" + base64.StdEncoding.EncodeToString([]byte(inner))
+}
+
 func TestParse_KcpShareParams(t *testing.T) {
 	// The emitter flattens one mkcp-legacy mask per field into headerType/seed; a merged
 	// mask drops the seed in xray-core (MkcpLegacy.Build), so import rebuilds them separately.
@@ -323,6 +328,20 @@ func TestParse_KcpShareParams(t *testing.T) {
 			wantMTU: 1350,
 			wantTTI: 20,
 		},
+		{
+			name:      "vmess header and seed from type and path",
+			link:      vmessKcpLink(`"type":"wechat-video","path":"secret-seed","mtu":1400,"tti":50`),
+			wantMTU:   1400,
+			wantTTI:   50,
+			wantMasks: []mkcpMask{{"", "secret-seed"}, {"wechat", ""}},
+		},
+		{
+			name:      "vmess fm mkcp-legacy mask wins over path",
+			link:      vmessKcpLink(`"type":"none","path":"s3cr3tSeed","mtu":1350,"tti":20,"fm":"{\"udp\":[{\"settings\":{\"header\":\"\",\"value\":\"s3cr3tSeed\"},\"type\":\"mkcp-legacy\"}]}"`),
+			wantMTU:   1350,
+			wantTTI:   20,
+			wantMasks: []mkcpMask{{"", "s3cr3tSeed"}},
+		},
 	}
 	for _, c := range cases {
 		t.Run(c.name, func(t *testing.T) {
@@ -340,3 +359,15 @@ func TestParse_KcpShareParams(t *testing.T) {
 		})
 	}
 }
+
+func TestParse_VmessFinalMask(t *testing.T) {
+	res, err := ParseLink(vmessKcpLink(`"type":"none","fm":"{\"udp\":[{\"settings\":{\"password\":\"obfspw123\"},\"type\":\"salamander\"}]}"`))
+	if err != nil {
+		t.Fatalf("parse: %v", err)
+	}
+	udp := finalmaskUDP(t, res)
+	want := []any{map[string]any{"type": "salamander", "settings": map[string]any{"password": "obfspw123"}}}
+	if !reflect.DeepEqual(udp, want) {
+		t.Fatalf("finalmask.udp = %#v, want %#v", udp, want)
+	}
+}