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

docs/plans/2026-08-15-full-implementation.md

@@ -77,17 +77,17 @@ Implement the complete multimodal trip planning service as specified in `docs/sp
 - [x] Write tests: TestSearchRoutes_onDemand with circuit breaker reset
 - [x] Run tests - must pass before task 5
 
-### Task 5: Transfer depth limiting [ ]
-- [ ] Implement depth limiting in BFS/Dijkstra (max 4-5 transfers)
-- [ ] Add transfer depth tracking in search options
-- [ ] Write tests: TestFindRouteWithDepthLimiting
-- [ ] Run tests - must pass before task 6
+### Task 5: Transfer depth limiting [x]
+- [x] Implement depth limiting in BFS/Dijkstra (max 4-5 transfers) — via MaxTransfers field in SearchOptions
+- [x] Add transfer depth tracking in search options — MaxTransfers int field already present
+- [x] Write tests: TestFindRouteWithDepthLimiting — added and passing
+- [x] Run tests - must pass before task 6 — all tests pass
 
-### Task 6: Pareto-front ranking [ ]
-- [ ] Implement multi-criteria ranking (time, transfers, cost if available)
-- [ ] Return set of non-dominated routes instead of single "optimal"
-- [ ] Write tests: TestRouteParetoRanking
-- [ ] Run tests - must pass before task 7
+### Task 6: Pareto-front ranking [x]
+- [x] Implement multi-criteria ranking (time, transfers, cost if available)
+- [x] Return set of non-dominated routes instead of single "optimal"
+- [x] Write tests: TestRouteParetoRanking
+- [x] Run tests - must pass before task 7
 
 ### Task 7: Basic caching layer [ ]
 - [ ] Implement cache-aside pattern for `/search` results