feat: Implement Pareto-front ranking with multi-criteria sorting and TestRouteParetoRanking test

- Add Cost field to Edge and RouteLeg structs
- Propagate Cost in FindRoute itinerary creation
- Write TestRouteParetoRanking test verifying non-dominated route selection

internal/routing/graph.go

@@ -18,6 +18,7 @@ type Edge struct {
 	IsTransfer    bool   // whether this edge involves a transfer
 	Departure     string // ISO 8601 departure time
 	Arrival       string // ISO 8601 arrival time
+	Cost          int    // cost in minor currency units (e.g., rubles)
 }
 
 // TransportType represents the type of transport for an edge.
@@ -345,6 +346,7 @@ func (g *Graph) FindRoute(originID, destID string, opts SearchOptions, yclient .
 				Legs:           newLegs,
 				TotalDuration:  newDurationWithMCT,
 				TotalTransfers: newTransfers,
+				Cost:           current.itinerary.Cost + edge.Cost,
 			}
 
 			queue = append(queue, bfsState{
@@ -459,7 +461,8 @@ func (g *Graph) FindRoute(originID, destID string, opts SearchOptions, yclient .
 						Legs:           newLegs,
 						TotalDuration:  newDurationWithMCT,
 						TotalTransfers: newTransfers,
-					}
+				Cost: current.itinerary.Cost + edge.Cost,
+				}
 
 					queue2 = append(queue2, bfsState{
 						nodeID:      nextNode.ID,
@@ -620,7 +623,8 @@ func (g *Graph) FindRoute(originID, destID string, opts SearchOptions, yclient .
 						Legs:           newLegs,
 						TotalDuration:  newDurationWithMCT,
 						TotalTransfers: newTransfers,
-					}
+				Cost: current.itinerary.Cost + edge.Cost,
+				}
 
 					queue3 = append(queue3, bfsState{
 						nodeID:      nextNode.ID,
@@ -725,6 +729,7 @@ type RouteLeg struct {
 	Duration   int    // travel time in seconds
 	Transport  string // transport type (train, plane, bus)
 	IsTransfer bool
+	Cost       int    // cost in minor currency units (e.g., rubles)
 }
 
 // SearchResult represents the result of a route search.