perf(go): simplify realtime plate lookup

docs/architecture/storage-minimal-contract.md

@@ -63,7 +63,7 @@
    - 状态：schema 使用 `PRIMARY KEY (protocol, vin)`，不保留自增 `id` 和 `created_at`，对外只暴露最新 `updated_at`。
    - 索引：不在 `vehicle_realtime_location` 维护经纬度组合索引；实时表只回答当前态，地理范围和轨迹类查询走 TDengine 历史位置。
    - 查询：`/api/realtime/snapshots`、`/api/realtime/locations` 默认不执行 `COUNT(*)`，`total` 表示本页返回条数；只有需要精确总数时传 `includeTotal=true`。
-   - 写入：车牌从 `vehicle_identity_binding` 反查时使用 VIN 级短 TTL 内存缓存，避免实时帧每条都打 MySQL；缓存不作为事实来源。
+   - 写入：车牌从 `vehicle_identity_binding` 按 VIN 主键直查，并使用 VIN 级短 TTL 内存缓存，避免实时帧每条都打 MySQL；缓存不作为事实来源。
    - 生产：项目上线前可直接重建这两张 MySQL 表，实时数据会从 Kafka 新消息继续投影。
 
 7. identity 层只保留两张表。

go/vehicle-gateway/internal/realtime/snapshot_writer.go

@@ -288,7 +288,7 @@ func (r *BindingPlateResolver) PlateByVIN(ctx context.Context, vin string) (stri
 	if vin == "" {
 		return "", sql.ErrNoRows
 	}
-	query := "SELECT plate FROM " + r.table + " WHERE vin = ? AND plate IS NOT NULL AND plate <> '' ORDER BY updated_at DESC LIMIT 1"
+	query := "SELECT plate FROM " + r.table + " WHERE vin = ? AND plate IS NOT NULL AND plate <> ''"
 	var plate string
 	err := r.queryer.QueryRowContext(ctx, query, vin).Scan(&plate)
 	if err != nil {

go/vehicle-gateway/internal/realtime/snapshot_writer_test.go

@@ -216,6 +216,28 @@ func TestSnapshotWriterBackfillsPlateFromBindingByVIN(t *testing.T) {
 	}
 }
 
+func TestBindingPlateResolverUsesPrimaryKeyLookupWithoutSort(t *testing.T) {
+	db, mock, err := sqlmock.New()
+	if err != nil {
+		t.Fatalf("sqlmock.New() error = %v", err)
+	}
+	defer db.Close()
+	mock.ExpectQuery("SELECT plate FROM vehicle_identity_binding WHERE vin = \\? AND plate IS NOT NULL AND plate <> ''$").
+		WithArgs("VIN001").
+		WillReturnRows(sqlmock.NewRows([]string{"plate"}).AddRow("沪A12345"))
+
+	plate, err := NewBindingPlateResolver(db, "vehicle_identity_binding").PlateByVIN(context.Background(), " VIN001 ")
+	if err != nil {
+		t.Fatalf("PlateByVIN() error = %v", err)
+	}
+	if plate != "沪A12345" {
+		t.Fatalf("plate = %q", plate)
+	}
+	if err := mock.ExpectationsWereMet(); err != nil {
+		t.Fatalf("sql expectations: %v", err)
+	}
+}
+
 func TestSnapshotWriterCachesBindingPlateByVIN(t *testing.T) {
 	exec := &recordingSnapshotExec{}
 	resolver := &recordingPlateResolver{plate: "沪A12345"}