docs: align jt808 mileage metric design

docs/superpowers/plans/2026-06-30-kafka-streaming-mileage.md

@@ -2,14 +2,14 @@
 
 ## Current Goal
 
-Consume JT808 Kafka location events, keep rolling daily state, and write the derived daily mileage into the common vehicle-stat metric repository.
+Consume JT808 Kafka location events and write the derived daily mileage into the common vehicle-stat metric repository.
 
 ## Current Design
 
 - Source topic: `vehicle.event.jt808.v1`
 - Runtime app: `vehicle-analytics-app`
-- Runtime state: Redis `Jt808MileageStateStore`
+- Runtime state: none outside `vehicle_stat_metric`
-- Metric output: `VehicleStatRepository.saveDailyStat(...)`
+- Metric output: `VehicleStatRepository.recordDailyMileageSample(...)`
 - Production metric storage: JDBC/MySQL `vehicle_stat_metric`
 - Date boundary: `Asia/Shanghai`
 - Calculation method: `JT808_TOTAL_MILEAGE_DIFF`
@@ -20,7 +20,14 @@ JT808 daily mileage is calculated only from the GPS total mileage reported by JT
 daily_mileage_km = last_total_mileage_km - first_total_mileage_km
 ```
 
-The stream saves a metric only when at least two ordered location points for the same vehicle and local day contain valid `total_mileage_km`. Negative differences are treated as invalid and are not written.
+The first valid local-day sample writes `daily_mileage_km=0.0`. Later samples update the same metric row with:
+
+```text
+metric_value = latest_total_mileage_km - first_total_mileage_km
+metric_key = daily_mileage_km
+```
+
+The first and latest GPS total mileage values stay on that same metric row as calculation source columns so restarts recover from MySQL without Redis or another state table.
 
 ## Runtime Properties
 
@@ -32,12 +39,8 @@ VEHICLE_STAT_REPOSITORY_TYPE=jdbc
 MYSQL_JDBC_URL=<jdbc-url>
 MYSQL_USERNAME=<user>
 MYSQL_PASSWORD=<password>
-REDIS_HOST=<host>
-REDIS_PORT=6379
-REDIS_DATABASE=50
-REDIS_PASSWORD=<password>
 ```
 
 ## Notes
 
-Do not create or write a protocol-specific JT808 daily-mileage table. Do not add GPS segment distance, speed integral, or jump-filter state back into this path unless the mileage definition changes again.
+Do not create or write a protocol-specific JT808 daily-mileage table. Do not add distance accumulation, integral calculation, Redis state, or memory state back into this path unless the mileage definition changes again.

modules/services/vehicle-stat-service/src/test/java/com/lingniu/ingest/vehiclestat/VehicleStatRepositoryContractTest.java

@@ -2,7 +2,10 @@ package com.lingniu.ingest.vehiclestat;
 
 import org.junit.jupiter.api.Test;
 
+import java.io.IOException;
 import java.lang.reflect.Method;
+import java.nio.file.Files;
+import java.nio.file.Path;
 import java.util.Arrays;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -15,4 +18,30 @@ class VehicleStatRepositoryContractTest {
                 .map(Method::getName))
                 .containsExactlyInAnyOrder("recordDailyMileageSample", "findDailyStat");
     }
+
+    @Test
+    void jt808MileagePlanDocumentsMetricTableDiffOnly() throws IOException {
+        String plan = Files.readString(repositoryRoot()
+                .resolve("docs/superpowers/plans/2026-06-30-kafka-streaming-mileage.md"));
+
+        assertThat(plan)
+                .contains("Runtime state: none outside `vehicle_stat_metric`")
+                .contains("daily_mileage_km = last_total_mileage_km - first_total_mileage_km")
+                .contains("Do not create or write a protocol-specific JT808 daily-mileage table.")
+                .doesNotContain("Redis `Jt808MileageStateStore`")
+                .doesNotContain("REDIS_HOST")
+                .doesNotContain("GPS segment distance")
+                .doesNotContain("speed integral");
+    }
+
+    private static Path repositoryRoot() {
+        Path path = Path.of("").toAbsolutePath();
+        while (path != null && !Files.exists(path.resolve("docs/superpowers/plans"))) {
+            path = path.getParent();
+        }
+        if (path == null) {
+            throw new IllegalStateException("repository root not found");
+        }
+        return path;
+    }
 }