router/semantic: baseline types, test helpers, leakage test (slice 13)

Baseline types: LegacyRouter interface (satisfied by *router.Router),
LegacyCase with PrerouteConsumed flag for command-prohibition detection,
LegacyReport with fast-path/residual/router-residual/pre-route breakdowns,
ContrastFamilyReport for per-transform-family scoring.

Test helpers: buildSeededClassifier (hash embedder seeded from
models/seeds/*.txt), actVerbList, seed file loading.

TestContrastFamiliesShareSplitGroup verifies that all contrastive
variants of one base seed share exactly one SplitGroup, preventing
train/eval leakage across the contrast family split.

internal/router/semantic/contract_test.go

@@ -287,3 +287,53 @@ func TestFamilyID(t *testing.T) {
 		t.Error("different base IDs produced same FamilyID")
 	}
 }
+
+func TestContrastFamiliesShareSplitGroup(t *testing.T) {
+	exs, err := LoadCorpus()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Group contrastive examples by their base source_id.
+	baseGroups := map[string]map[string]bool{} // source_id → {split_group: true}
+	for _, e := range exs {
+		if e.Source != "contrastive" {
+			continue
+		}
+		if baseGroups[e.SourceID] == nil {
+			baseGroups[e.SourceID] = map[string]bool{}
+		}
+		baseGroups[e.SourceID][e.SplitGroup] = true
+	}
+
+	// Every base seed's contrastive variants must share exactly one
+	// split_group (the base seed's own group).
+	for sourceID, groups := range baseGroups {
+		if len(groups) != 1 {
+			t.Errorf("base %q has contrastive variants in %d split groups: %v",
+				sourceID, len(groups), groups)
+		}
+	}
+
+	// Also verify that contrastive variants share the split_group with
+	// their base seed from ru_routing_v1.
+	ruGroups := map[string]string{} // source_id → split_group
+	for _, e := range exs {
+		if e.Source == "ru_routing_v1" {
+			ruGroups[e.SourceID] = e.SplitGroup
+		}
+	}
+	for sourceID, groups := range baseGroups {
+		var contrastGroup string
+		for g := range groups {
+			contrastGroup = g
+		}
+		if ruGroup, ok := ruGroups[sourceID]; ok && contrastGroup != ruGroup {
+			t.Errorf("base %q: ru_routing_v1 split_group=%q, contrastive split_group=%q",
+				sourceID, ruGroup, contrastGroup)
+		}
+	}
+
+	t.Logf("contrast family leakage check passed: %d base seeds, all groups consistent",
+		len(baseGroups))
+}