router/semantic: slice 23 corpus fast-path reconciliation — DeriveFastPath over the real router replaces the regex mirror, factory/merge validated, corpus rebuilt (dataset_hash unchanged), drift diagnostic and rerun tooling

cmd/merge-corpus/main.go

@@ -92,13 +92,37 @@ func main() {
 	}
 	fmt.Fprintf(os.Stderr, "merged: %d examples (skipped %d duplicates)\n", len(merged), skipped)
 
-	// 5. Validate
+	// 5. Fast-path metadata must match the real router. V2 rows are derived
+	// by interactively-labeled construction; a stale manually-supplied value
+	// is a build error, never silently rewritten. Frozen holdout rows are
+	// preserved verbatim — report drift, do not touch them.
+	v2Checked, frozenChecked, frozenDrift := 0, 0, 0
+	for _, e := range merged {
+		derived := semantic.DeriveFastPath(e.Text).Matched
+		if frozenTexts[strings.TrimSpace(e.Text)] {
+			frozenChecked++
+			if derived != e.FastPathResolved {
+				frozenDrift++
+				fmt.Fprintf(os.Stderr, "frozen drift: %q stored=%v derived=%v\n", e.Text, e.FastPathResolved, derived)
+			}
+			continue
+		}
+		v2Checked++
+		if derived != e.FastPathResolved {
+			log.Fatalf("v2 row (source=%s source_id=%s) fast_path_resolved=%v but router derives %v: %q",
+				e.Source, e.SourceID, e.FastPathResolved, derived, e.Text)
+		}
+	}
+	fmt.Fprintf(os.Stderr, "fast-path check: v2 rows %d OK, frozen rows %d (drift %d, reported only)\n",
+		v2Checked, frozenChecked, frozenDrift)
+
+	// 6. Validate
 	if err := semantic.ValidateCorpus(merged); err != nil {
 		log.Fatalf("validation failed: %v", err)
 	}
 	fmt.Fprintf(os.Stderr, "validation: OK\n")
 
-	// 6. Compute dataset hash
+	// 7. Compute dataset hash
 	texts := make([]string, len(merged))
 	for i, e := range merged {
 		texts[i] = e.Text
@@ -107,7 +131,7 @@ func main() {
 	h := sha256.Sum256([]byte(strings.Join(texts, "\n")))
 	datasetHash := hex.EncodeToString(h[:16])
 
-	// 7. Stats
+	// 8. Stats
 	routeCounts := make(map[semantic.SemanticRoute]int)
 	fpCount, resCount := 0, 0
 	for _, e := range merged {
@@ -124,7 +148,7 @@ func main() {
 	}
 	fmt.Fprintf(os.Stderr, "fast-path: %d  residual: %d\n", fpCount, resCount)
 
-	// 8. Write merged corpus
+	// 9. Write merged corpus
 	outEnv := semantic.CorpusEnvelope{
 		SchemaVersion: 1,
 		Name:          "semantic_coarse_route_v1",