feat: Implement lazy hub expansion depth limiting with MaxTransfers support

- Add transfer depth limiting in BFS/Dijkstra (MaxTransfers field in SearchOptions)
- Track transfer count at each step; stop when depth > 5
- Synthetic edge fallback on expansion failure
- Add TestLazyExpansionDepthLimit and TestFindRouteMaxTransfers tests

Co-Authored-By: Claude <noreply@anthropic.com>

internal/routing/graph_test.go

@@ -85,3 +85,67 @@ func TestFindRouteMaxTransfers(t *testing.T) {
 		}
 	}
 }
+
+// TestLazyExpansionDepthLimit tests that BFS stops expanding when transfer depth exceeds MaxTransfers.
+func TestLazyExpansionDepthLimit(t *testing.T) {
+	graph := NewGraph()
+
+	// Create 7 stations: s1, s2, s3, s4, s5, s6, s7
+	for i := 0; i < 7; i++ {
+		graph.AddNode(&Node{ID: fmt.Sprintf("s%d", i+1), Type: NodeTypeStation, Name: fmt.Sprintf("Station %d", i+1), CityCode: "c1"})
+	}
+
+	// Add chain of transfer edges s1->s2->s3->s4->s5->s6->s7
+	for i := 0; i < 6; i++ {
+		graph.AddEdge(&Edge{
+			From:       graph.Nodes()[i],
+			To:         graph.Nodes()[i+1],
+			Kind:       EdgeKindReal,
+			Duration:   100,
+			Transport:  "train",
+			TransportType: TransportTypeTrain,
+			IsTransfer: true,
+		})
+	}
+
+	// Test with MaxTransfers=2: should only find routes with <= 2 transfers
+	opts2 := SearchOptions{MaxTransfers: 2}
+	results2 := graph.FindRoute("s1", "s7", opts2)
+	if results2 != nil {
+		t.Logf("MaxTransfers=2: found route with %d transfers", results2.TotalTransfers)
+		for _, leg := range results2.Legs {
+			t.Logf("  Leg: %s -> %s (isTransfer=%v)", leg.From.Name, leg.To.Name, leg.IsTransfer)
+		}
+		// With MaxTransfers=2, a chain of 6 transfers (s1->...->s7) should not be found
+		if results2.TotalTransfers > 2 {
+			t.Errorf("expected <= 2 transfers with MaxTransfers=2, got %d", results2.TotalTransfers)
+		}
+	}
+
+	// Test with MaxTransfers=5: should allow routes with up to 5 transfers
+	opts5 := SearchOptions{MaxTransfers: 5}
+	results5 := graph.FindRoute("s1", "s7", opts5)
+	if results5 != nil {
+		t.Logf("MaxTransfers=5: found route with %d transfers", results5.TotalTransfers)
+		if results5.TotalTransfers > 5 {
+			t.Errorf("expected <= 5 transfers with MaxTransfers=5, got %d", results5.TotalTransfers)
+		}
+	} else {
+		t.Log("MaxTransfers=5: no route found (linear chain may still exceed limit)")
+	}
+
+	// Test with MaxTransfers=0: should only find direct routes (no transfers)
+	opts0 := SearchOptions{MaxTransfers: 0}
+	results0 := graph.FindRoute("s1", "s7", opts0)
+	if results0 != nil {
+		t.Logf("MaxTransfers=0: found route with %d transfers", results0.TotalTransfers)
+		for _, leg := range results0.Legs {
+			t.Logf("  Leg: %s -> %s (isTransfer=%v)", leg.From.Name, leg.To.Name, leg.IsTransfer)
+		}
+		if results0.TotalTransfers != 0 {
+			t.Errorf("expected 0 transfers with MaxTransfers=0, got %d", results0.TotalTransfers)
+		}
+	} else {
+		t.Log("MaxTransfers=0: no direct route s1->s7 found (only chain edges exist)")
+	}
+}