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.go

@@ -330,13 +330,6 @@ func (g *Graph) FindRoute(originID, destID string, opts SearchOptions, yclient .
 			continue
 		}
 
-		// Prune if we've exceeded max transfers
-		// Use strict > comparison: with MaxTransfers=5, transfers 0-5 are allowed,
-		// and we stop when transfers would exceed the limit (depth > 5)
-		if opts.MaxTransfers >= 0 && current.transfers > opts.MaxTransfers {
-			continue
-		}
-
 		// Explore outgoing edges
 		for _, edge := range adj[current.nodeID] {
 			nextNode := edge.To
@@ -398,6 +391,11 @@ func (g *Graph) FindRoute(originID, destID string, opts SearchOptions, yclient .
 				Cost:           current.itinerary.Cost + edge.Cost,
 			}
 
+			// Skip this edge if it would exceed the maximum allowed transfers
+			if opts.MaxTransfers >= 0 && newTransfers > opts.MaxTransfers {
+				continue
+			}
+
 			queue = append(queue, bfsState{
 				nodeID:      nextNode.ID,
 				transfers:   newTransfers,
@@ -513,6 +511,10 @@ func (g *Graph) FindRoute(originID, destID string, opts SearchOptions, yclient .
 				Cost: current.itinerary.Cost + edge.Cost,
 				}
 
+							if opts.MaxTransfers >= 0 && newTransfers > opts.MaxTransfers {
+							continue
+						}
+
 					queue2 = append(queue2, bfsState{
 						nodeID:      nextNode.ID,
 						transfers:   newTransfers,
@@ -676,6 +678,10 @@ func (g *Graph) FindRoute(originID, destID string, opts SearchOptions, yclient .
 				Cost: current.itinerary.Cost + edge.Cost,
 				}
 
+							if opts.MaxTransfers >= 0 && newTransfers > opts.MaxTransfers {
+							continue
+						}
+
 					queue3 = append(queue3, bfsState{
 						nodeID:      nextNode.ID,
 						transfers:   newTransfers,