perf(go): simplify identity binding lookup

docs/architecture/storage-minimal-contract.md

@@ -70,6 +70,7 @@
    - `vehicle_identity_binding`：VIN 与 plate/phone/device_id 的映射，供 808 等协议反查 VIN。
    - `jt808_registration`：808 注册、鉴权、最新活跃和 VIN 匹配状态。
    - 状态：`vehicle_identity_binding` 使用 VIN 主键；`jt808_registration` 使用 phone 主键；两张表都不保留代理自增主键和 `created_at`。
+   - 查询：`vehicle_identity_binding` 的 phone/device_id/plate 都是唯一键，接入侧按唯一键直查 VIN，不做无意义排序。
    - 生产：RDS 已在上线前删除旧 `vehicle_identity_binding_registration`、`vehicle_identity_bindings`，gateway 启动会自动创建最小 schema。
 
 ## 字段提升规则

go/vehicle-gateway/internal/identity/resolver.go

@@ -161,7 +161,7 @@ func (r *MySQLResolver) Resolve(ctx context.Context, env envelope.FrameEnvelope)
 }
 
 func (r *MySQLResolver) lookup(ctx context.Context, column string, value string) (string, error) {
-	query := "SELECT vin FROM " + r.table + " WHERE " + column + " = ? AND vin IS NOT NULL AND vin <> '' ORDER BY updated_at DESC LIMIT 1"
+	query := "SELECT vin FROM " + r.table + " WHERE " + column + " = ? AND vin IS NOT NULL AND vin <> ''"
 	var vin string
 	err := r.db.QueryRowContext(ctx, query, value).Scan(&vin)
 	return vin, err

go/vehicle-gateway/internal/identity/resolver_test.go

@@ -80,6 +80,30 @@ func TestMySQLResolverFillsVINFromPhone(t *testing.T) {
 	}
 }
 
+func TestMySQLResolverLooksUpVINByUniqueKeyWithoutSort(t *testing.T) {
+	db, mock := newMockDB(t)
+	defer db.Close()
+	mock.ExpectQuery("SELECT vin FROM vehicle_identity_binding WHERE phone = \\? AND vin IS NOT NULL AND vin <> ''$").
+		WithArgs("13307795425").
+		WillReturnRows(sqlmock.NewRows([]string{"vin"}).AddRow("LNBVIN00000000001"))
+
+	resolver := NewMySQLResolver(db, "vehicle_identity_binding")
+	env, err := resolver.Resolve(context.Background(), envelope.FrameEnvelope{
+		Protocol: envelope.ProtocolJT808,
+		Phone:    "013307795425",
+		Parsed:   map[string]any{},
+	})
+	if err != nil {
+		t.Fatalf("Resolve() error = %v", err)
+	}
+	if env.VIN != "LNBVIN00000000001" {
+		t.Fatalf("vin = %q", env.VIN)
+	}
+	if err := mock.ExpectationsWereMet(); err != nil {
+		t.Fatalf("sql expectations: %v", err)
+	}
+}
+
 func TestMySQLResolverFallsBackToDeviceID(t *testing.T) {
 	db, mock := newMockDB(t)
 	defer db.Close()