feat: implement synthetic edge fallback in FindRoute

- Add addSyntheticEdgesForNode function that connects nodes to city hubs
- Modify FindRoute to add synthetic edges as fallback when no route found
- Write TestFindRouteWithSyntheticFallback test

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

internal/routing/graph_test.go

@@ -544,6 +544,58 @@ func TestSortEdges_AlreadySorted(t *testing.T) {
 	}
 }
 
+// TestSelectHubStations tests the SelectHubStations function.
+// It verifies that hub stations are selected based on the minOutgoingFlights criterion.
+func TestSelectHubStations(t *testing.T) {
+	// Test with stations from different cities
+	stations := []StationInfo{
+		{ID: "s1", Name: "Station 1", CityCode: "c1", CityName: "City A"},
+		{ID: "s2", Name: "Station 2", CityCode: "c2", CityName: "City B"},
+		{ID: "s3", Name: "Station 3", CityCode: "c3", CityName: "City C"},
+		{ID: "s4", Name: "Station 4", CityCode: "c4", CityName: "City D"},
+	}
+
+	// With minOutgoingFlights=2, all 4 stations connect to 4 unique cities, so all should be selected
+	hubs := SelectHubStations(stations, 2)
+	if len(hubs) != 4 {
+		t.Errorf("expected 4 hubs with minOutgoingFlights=2, got %d", len(hubs))
+	}
+
+	// Verify all stations are included
+	ids := make(map[string]bool)
+	for _, h := range hubs {
+		ids[h.ID] = true
+	}
+	if !ids["s1"] || !ids["s2"] || !ids["s3"] || !ids["s4"] {
+		t.Error("expected all 4 stations to be selected as hubs")
+	}
+
+	// With minOutgoingFlights=5, only stations with 5+ unique city connections should be selected
+	// There are only 4 unique cities, so no stations should be selected
+	hubs5 := SelectHubStations(stations, 5)
+	if len(hubs5) != 0 {
+		t.Errorf("expected 0 hubs with minOutgoingFlights=5, got %d", len(hubs5))
+	}
+
+	// With minOutgoingFlights=1, all stations should be selected (1+ cities)
+	hubs1 := SelectHubStations(stations, 1)
+	if len(hubs1) != 4 {
+		t.Errorf("expected 4 hubs with minOutgoingFlights=1, got %d", len(hubs1))
+	}
+
+	// Edge case: empty stations list
+	hubsEmpty := SelectHubStations(nil, 1)
+	if len(hubsEmpty) != 0 {
+		t.Errorf("expected 0 hubs for empty stations list, got %d", len(hubsEmpty))
+	}
+
+	// Edge case: empty stations list with 0 min outgoing flights
+	hubsZero := SelectHubStations(nil, 0)
+	if len(hubsZero) != 0 {
+		t.Errorf("expected 0 hubs for nil stations with minOutgoingFlights=0, got %d", len(hubsZero))
+	}
+}
+
 // TestSortEdges_ReverseSorted tests that reverse-sorted edges are correctly sorted.
 func TestSortEdges_ReverseSorted(t *testing.T) {
 	edges := []*Edge{
@@ -556,3 +608,44 @@ func TestSortEdges_ReverseSorted(t *testing.T) {
 		t.Error("expected edges to be sorted from shortest to longest")
 	}
 }
+
+// TestFindRouteWithSyntheticFallback tests that FindRoute can find routes
+// via synthetic edges when lazy expansion finds no direct connection.
+func TestFindRouteWithSyntheticFallback(t *testing.T) {
+	// Create a graph using BuildGraphFromStations with stations in the same city
+	stations := []StationInfo{
+		{ID: "s1", Name: "Moscow", CityCode: "c1", CityName: "Moscow"},
+		{ID: "s2", Name: "Tula", CityCode: "c1", CityName: "Moscow"},
+	}
+	graph := BuildGraphFromStations(stations)
+
+	// Search for route with max 2 transfers between the two stations
+	// They're connected via the city hub with synthetic edges (s1 -> city_hub -> s2)
+	opts := SearchOptions{MaxTransfers: 2, MCT: 300}
+	result := graph.FindRoute("s1", "s2", opts)
+
+	// Should find a route via synthetic edges (s1 -> city_hub -> s2)
+	if result == nil {
+		t.Error("expected a route to be found via synthetic edges")
+	}
+	// 2 synthetic edges: s1->city_hub and city_hub->s2, each IsTransfer=true
+	if result.TotalTransfers != 2 {
+		t.Errorf("expected 2 transfers (via city hub), got %d", result.TotalTransfers)
+	}
+	if result.TotalDuration <= 0 {
+		t.Errorf("expected positive duration, got %d", result.TotalDuration)
+	}
+
+	// Verify synthetic edges exist in the graph
+	edges := graph.Edges()
+	syntheticCount := 0
+	for _, e := range edges {
+		if e.Kind == EdgeKindSynthetic {
+			syntheticCount++
+		}
+	}
+	// BuildGraphFromStations adds 2 synthetic edges (station<->city hub) per station = 4 total
+	if syntheticCount < 4 {
+		t.Errorf("expected at least 4 synthetic edges from BuildGraphFromStations, got %d", syntheticCount)
+	}
+}