docs: align jt808 mileage metric design

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;
+    }
 }