feat(stats): store source mileage candidates

.superpowers/sdd/task-2-report.md

@@ -0,0 +1,52 @@
+# Task 2 Report: Candidate Mileage Schema And Writer
+
+## Outcome
+- Implemented Task 2 in `go/vehicle-gateway/internal/stats`.
+- Added candidate mileage schema, sample-to-candidate mapping, and the upsert writer.
+
+## RED Evidence
+The focused candidate tests failed before implementation because the new symbols did not exist:
+
+```bash
+cd /Users/lingniu/project/ai-coding/lingniu-vehicle-ingest/go/vehicle-gateway
+go test ./internal/stats -run 'TestSourceKey|TestUpsertSourceMileage' -count=1
+```
+
+Result:
+- `undefined: SourceKey`
+- `undefined: SourceMileageSample`
+- `undefined: QualityOK`
+- `undefined: UpsertSourceMileage`
+
+## GREEN Evidence
+After implementation, the focused candidate tests passed:
+
+```bash
+cd /Users/lingniu/project/ai-coding/lingniu-vehicle-ingest/go/vehicle-gateway
+go test ./internal/stats -run 'TestSourceKey|TestUpsertSourceMileage' -count=1
+```
+
+Result:
+- `ok  	lingniu-vehicle-ingest/go/vehicle-gateway/internal/stats	0.556s`
+
+Package verification also passed:
+
+```bash
+cd /Users/lingniu/project/ai-coding/lingniu-vehicle-ingest/go/vehicle-gateway
+go test ./internal/stats -count=1
+```
+
+Result:
+- `ok  	lingniu-vehicle-ingest/go/vehicle-gateway/internal/stats	0.235s`
+
+## Files Changed
+- `go/vehicle-gateway/internal/stats/schema.go`
+- `go/vehicle-gateway/internal/stats/daily_metric.go`
+- `go/vehicle-gateway/internal/stats/source_mileage.go`
+- `go/vehicle-gateway/internal/stats/source_mileage_test.go`
+
+## Self-Review
+- The candidate schema matches the brief's table shape and indexes.
+- `SamplesFromEnvelope` now carries `EventTime` and `DeviceID`, which the new candidate mapping needs.
+- The upsert SQL is focused on the candidate table and reuses the shared `Execer` interface.
+- No unrelated stats files were modified.