feat: Implement synthetic edges city↔airport (Task 10)

- Add TransferTime constants (AirportToCity, CityToStation, StationToStation)
- Update Edge struct with Synthetic field
- Enhance addSyntheticEdgesForNode to use constants and mark synthetic edges
- Update BuildGraphFromStations to set Synthetic field
- Mark synthetic edges in GeoJSON output as dashed lines
- Write TestSyntheticAirportCityEdges and TestRouteWithSyntheticAirportCityEdges tests

internal/routing/graph_test.go

@@ -451,4 +451,117 @@ func TestRouteParetoRanking(t *testing.T) {
 			}
 		}
 	}
-}
+}
+// TestSyntheticAirportCityEdges tests that synthetic edges are correctly created
+// for airport-city transfers, including proper transport type and transfer time constants.
+func TestSyntheticAirportCityEdges(t *testing.T) {
+	// Test 1: Synthetic edges from station to airport city hub
+	graph := NewGraph()
+	graph.AddNode(&Node{ID: "s1", Type: NodeTypeStation, Name: "Moscow", CityCode: "c_airport"})
+	graph.AddNode(&Node{ID: "c1", Type: NodeTypeCity, Name: "Airport City", CityCode: "c_airport"})
+
+	// Add synthetic edges via the function
+	addSyntheticEdgesForNode(graph, graph.Nodes()[0])
+
+	edges := graph.Edges()
+	if len(edges) != 2 {
+		t.Errorf("expected 2 synthetic edges (node->city and city->node), got %d", len(edges))
+	}
+
+	// Check that edges have correct transport type (Plane for airport)
+	for _, edge := range edges {
+		if edge.TransportType != TransportTypePlane {
+			t.Errorf("expected TransportTypePlane for airport edge, got %v", edge.TransportType)
+		}
+		if edge.Transport != "plane" {
+			t.Errorf("expected Transport 'plane', got %s", edge.Transport)
+		}
+		if !edge.Synthetic {
+			t.Error("expected edge to be marked as Synthetic")
+		}
+		if edge.Kind != EdgeKindSynthetic {
+			t.Error("expected edge Kind to be EdgeKindSynthetic")
+		}
+	}
+
+	// Test 2: Synthetic edges from station to regular city hub (train)
+	graph2 := NewGraph()
+	graph2.AddNode(&Node{ID: "s2", Type: NodeTypeStation, Name: "Moscow", CityCode: "c1"})
+	graph2.AddNode(&Node{ID: "c2", Type: NodeTypeCity, Name: "Regular City", CityCode: "c1"})
+
+	addSyntheticEdgesForNode(graph2, graph2.Nodes()[0])
+
+	edges2 := graph2.Edges()
+	if len(edges2) != 2 {
+		t.Errorf("expected 2 synthetic edges for regular city, got %d", len(edges2))
+	}
+
+	for _, edge := range edges2 {
+		if edge.TransportType != TransportTypeTrain {
+			t.Errorf("expected TransportTypeTrain for regular city edge, got %v", edge.TransportType)
+		}
+		if !edge.Synthetic {
+			t.Error("expected edge to be marked as Synthetic")
+		}
+	}
+
+	// Test 3: Verify transfer time constants
+	if AirportToCity != 5400 {
+		t.Errorf("expected AirportToCity constant to be 5400 (90 min), got %d", AirportToCity)
+	}
+	if CityToStation != 300 {
+		t.Errorf("expected CityToStation constant to be 300 (5 min), got %d", CityToStation)
+	}
+	if StationToStation != 300 {
+		t.Errorf("expected StationToStation constant to be 300 (5 min), got %d", StationToStation)
+	}
+}
+
+// TestRouteWithSyntheticAirportCityEdges tests that FindRoute correctly uses
+// synthetic airport-city edges when no direct route exists.
+func TestRouteWithSyntheticAirportCityEdges(t *testing.T) {
+	graph := NewGraph()
+
+	// Add airport station and city hub
+	graph.AddNode(&Node{ID: "s1", Type: NodeTypeStation, Name: "Sheremetyevo", CityCode: "c_airport"})
+	graph.AddNode(&Node{ID: "c1", Type: NodeTypeCity, Name: "Moscow", CityCode: "c_airport"})
+
+	// Add synthetic edges (this normally happens via addSyntheticEdgesForNode or BuildGraphFromStations)
+	graph.AddEdge(&Edge{
+		From:       graph.Nodes()[0], // s1 Sheremetyevo
+		To:         graph.Nodes()[1], // c1 Moscow city
+		Kind:       EdgeKindSynthetic,
+		Duration:   AirportToCity,
+		Transport:  "plane",
+		TransportType: TransportTypePlane,
+		IsTransfer: true,
+		Synthetic:  true,
+	})
+	graph.AddEdge(&Edge{
+		From:       graph.Nodes()[1], // c1 Moscow
+		To:         graph.Nodes()[0], // s1 Sheremetyevo
+		Kind:       EdgeKindSynthetic,
+		Duration:   AirportToCity,
+		Transport:  "plane",
+		TransportType: TransportTypePlane,
+		IsTransfer: true,
+		Synthetic:  true,
+	})
+
+	// Search for route from Sheremetyevo to Moscow (should use synthetic edge)
+	opts := SearchOptions{MaxTransfers: 3, MCT: 300}
+	results := graph.FindRoutesPareto("s1", "c1", opts)
+
+	if len(results) == 0 {
+		t.Error("expected at least 1 route using synthetic airport-city edge")
+	}
+
+	// Verify the route uses the synthetic edge
+	for _, r := range results {
+		t.Logf("Route: duration=%d, transfers=%d, cost=%d", r.TotalDuration, r.TotalTransfers, r.Cost)
+		if r.TotalDuration < 5400 {
+			t.Logf("WARNING: Route duration %d is less than expected airport-to-city transfer %d",
+				r.TotalDuration, AirportToCity)
+		}
+	}
+}