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.go

@@ -19,6 +19,9 @@ type Edge struct {
 	Departure     string // ISO 8601 departure time
 	Arrival       string // ISO 8601 arrival time
 	Cost          int    // cost in minor currency units (e.g., rubles)
+	// Synthetic indicates whether this edge is a synthetic transfer edge
+	// (e.g., city↔airport, station↔city hub) rather than a real scheduled trip.
+	Synthetic bool
 }
 
 // TransportType represents the type of transport for an edge.
@@ -33,6 +36,19 @@ const (
 	TransportTypeBus TransportType = "bus"
 )
 
+// TransferTime constants for synthetic edge duration estimation.
+const (
+	// AirportToCity is the standard transfer time (in seconds) for airport-to-city
+	// or city-to-airport synthetic edges.
+	AirportToCity = 5400 // 90 minutes
+	// CityToStation is the standard transfer time (in seconds) for city-to-station
+	// or station-to-city synthetic edges within the same city.
+	CityToStation = 300 // 5 minutes
+	// StationToStation is the standard transfer time (in seconds) for station-to-station
+	// transfers within the same city.
+	StationToStation = 300 // 5 minutes
+)
+
 // NodeType represents the type of a graph node.
 type NodeType int
 
@@ -155,6 +171,7 @@ func BuildGraphFromStations(stations []StationInfo) *Graph {
 			Transport:  string(tp),
 			TransportType: tp,
 			IsTransfer: true,
+			Synthetic:  true,
 		})
 
 		// Add reverse synthetic edge: city hub -> station
@@ -166,6 +183,7 @@ func BuildGraphFromStations(stations []StationInfo) *Graph {
 			Transport:  string(tp),
 			TransportType: tp,
 			IsTransfer: true,
+			Synthetic:  true,
 		})
 	}
 
@@ -174,6 +192,7 @@ func BuildGraphFromStations(stations []StationInfo) *Graph {
 
 // addSyntheticEdgesForNode adds synthetic edges from the given node to city hubs
 // in the same city, as a fallback when direct route search fails.
+// Uses transfer time constants for duration estimation.
 func addSyntheticEdgesForNode(graph *Graph, node *Node) {
 	// Connect this node to city hubs in the same city via synthetic edges
 	for _, n := range graph.Nodes() {
@@ -185,15 +204,25 @@ func addSyntheticEdgesForNode(graph *Graph, node *Node) {
 			} else if node.CityCode == "c_bus" {
 				tp = TransportTypeBus
 			}
+
+			// Use appropriate transfer time constant based on node and city types
+			var duration int
+			if node.CityCode == "c_airport" {
+				duration = AirportToCity
+			} else {
+				duration = CityToStation
+			}
+
 			// Add synthetic edge from node to city hub
 			graph.AddEdge(&Edge{
 				From:       node,
 				To:         n,
 				Kind:       EdgeKindSynthetic,
-				Duration:   300, // 5 min synthetic transfer
+				Duration:   duration,
 				Transport:  string(tp),
 				TransportType: tp,
 				IsTransfer: true,
+				Synthetic:  true,
 			})
 
 			// Add reverse synthetic edge from city hub to node
@@ -201,10 +230,11 @@ func addSyntheticEdgesForNode(graph *Graph, node *Node) {
 				From:       n,
 				To:         node,
 				Kind:       EdgeKindSynthetic,
-				Duration:   300, // 5 min synthetic transfer
+				Duration:   duration,
 				Transport:  string(tp),
 				TransportType: tp,
 				IsTransfer: true,
+				Synthetic:  true,
 			})
 		}
 	}