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>

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

@@ -163,11 +163,11 @@ Implement the complete multimodal trip planning service as specified in `docs/sp
 ## Implementation Steps
 
 ### Task 14: Lazy hub expansion depth 4-5 [x]
-- [ ] Implement BFS/Dijkstra with explicit depth limiting
-- [ ] Track transfer count at each step; stop when depth > 5
-- [ ] On expansion failure, add synthetic edges as fallback
-- [ ] Write tests: TestLazyExpansionDepthLimit, TestFindRouteMaxTransfers
-- [ ] Run tests - must pass before task 15
+- [x] Implement BFS/Dijkstra with explicit depth limiting
+- [x] Track transfer count at each step; stop when depth > 5
+- [x] On expansion failure, add synthetic edges as fallback
+- [x] Write tests: TestLazyExpansionDepthLimit, TestFindRouteMaxTransfers
+- [x] Run tests - must pass before task 15
 
 ### Task 15: Pareto-front ranking integration [ ]
 - [ ] Integrate multi-criteria ranking into route search results