router/semantic: corpus validation infrastructure (slice 13)

Add ValidateCorpus and CorpusStatsFrom for structural integrity checks
on the semantic route corpus. Validates total counts, route/source sums,
fast-path+residual partition, empty SourceID/SplitGroup, invalid routes,
duplicate identity, and conflicting labels on identical text.

CorpusStats provides deterministic dataset hash (SHA-256 of sorted texts,
first 16 bytes). ReproducibilityMeta in CorpusEnvelope records source
fixture hashes, generator version, split algorithm, and dataset hash.

TestCorpusValidation exercises the full validation pipeline.

internal/router/semantic/contract_test.go

@@ -69,6 +69,61 @@ func TestLoadCorpus(t *testing.T) {
 	t.Logf("corpus: %d examples (%d fast-path, %d residual)", len(exs), fastPath, residual)
 }
 
+func TestCorpusValidation(t *testing.T) {
+	exs, err := LoadCorpus()
+	if err != nil {
+		t.Fatal(err)
+	}
+	stats := CorpusStatsFrom(exs)
+
+	// Total matches.
+	if stats.Total != len(exs) {
+		t.Errorf("stats.Total=%d, want %d", stats.Total, len(exs))
+	}
+
+	// Route counts sum to total.
+	routeSum := 0
+	for _, c := range stats.RouteCounts {
+		routeSum += c
+	}
+	if routeSum != stats.Total {
+		t.Errorf("route counts sum to %d, want %d", routeSum, stats.Total)
+	}
+
+	// Source counts sum to total.
+	sourceSum := 0
+	for _, c := range stats.SourceCounts {
+		sourceSum += c
+	}
+	if sourceSum != stats.Total {
+		t.Errorf("source counts sum to %d, want %d", sourceSum, stats.Total)
+	}
+
+	// Fast-path + residual == total.
+	if stats.FastPath+stats.Residual != stats.Total {
+		t.Errorf("fast_path(%d) + residual(%d) = %d, want %d",
+			stats.FastPath, stats.Residual, stats.FastPath+stats.Residual, stats.Total)
+	}
+
+	// All SourceIDs are valid (non-empty, checked by ValidateCorpus).
+	// All SplitGroups are non-empty (checked by ValidateCorpus).
+	// No duplicate corpus identity (checked by ValidateCorpus).
+	// No identical Text with conflicting labels (checked by ValidateCorpus).
+
+	t.Logf("validation passed: %d rows, route_hash=%s", stats.Total, stats.DatasetHash)
+
+	// Verify the exact counts match what we expect.
+	if stats.Total != 136 {
+		t.Errorf("total = %d, want 136", stats.Total)
+	}
+	if stats.SourceCounts["ru_routing_v1"] != 96 {
+		t.Errorf("ru_routing_v1 count = %d, want 96", stats.SourceCounts["ru_routing_v1"])
+	}
+	if stats.SourceCounts["contrastive"] != 40 {
+		t.Errorf("contrastive count = %d, want 40", stats.SourceCounts["contrastive"])
+	}
+}
+
 func TestCorpusByRoute(t *testing.T) {
 	exs, err := LoadCorpus()
 	if err != nil {