perf(go): start realtime projector at latest offset

docs/architecture/iot-data-platform-principles.md

@@ -71,7 +71,7 @@ Every received frame should become one `FrameEnvelope`.
 1. Stabilize ingress: connection lifecycle, protocol parser correctness, bounded backpressure, durable publish.
 2. Stabilize event log: NATS to Kafka bridge, topic names, partition keys, retry and replay.
 3. Simplify storage: keep only the minimal tables above, remove duplicated payloads from business tables.
-4. Stabilize realtime: Redis full latest raw state, MySQL lightweight snapshot and location.
+4. Stabilize realtime: Redis full latest raw state, MySQL lightweight snapshot and location. Realtime consumers start from latest when no committed offset exists; historical rebuilds must be explicit replay jobs, not accidental backlog scans.
 5. Stabilize history: raw and location query pagination, clear time zone behavior.
 6. Stabilize metrics: idempotent daily metrics from mileage differences and raw replay.
 7. Add operations: health, readiness, metrics, lag, connection counts, deploy notes.

docs/ops/vehicle-ingest-runbook.md

@@ -38,6 +38,8 @@
 
 如果 stat-writer 少消费某个 raw topic，对应协议的每日里程不会进入 `vehicle_daily_mileage`。修正后可能出现短时间 Kafka lag，这是在追补历史 backlog；只要 `vehicle_stat_writes_total` 持续增长且 lag 下降，就是健康状态。
 
+Realtime API 是当前态投影，默认在没有已提交 offset 时从 latest 开始消费。切换 topic 或新建 consumer group 后，不应让 realtime 追扫历史 raw backlog；需要重建当前态时，应使用明确的回放任务或手动 reset offset。
+
 业务端口：
 
 - GB32960 TCP：`0.0.0.0:32960`