TransferPathLeg exposes the wrong toStop in reverse searches

[t2gran]

Summary

TransferPathLeg derived its toStop from transfer.stop(), but that is direction-relative: a transfer obtained through
RaptorTransitDataProvider#getTransfersToStop(int) (the reverse-search path) reports stop() as the leg's origin, not its destination. The correct, direction-independent destination stop was already computed and threaded through PathBuilderLeg (every caller of PathBuilder#transfer(transfer, toStop) already passes it explicitly - see ForwardPathMapper and ReversePathMapper), but PathBuilderLeg#createTransferPathLeg() never used it, letting TransferPathLeg's constructor silently re-derive the wrong value from transfer.stop() instead.

TransferPathLeg now takes toStop as an explicit constructor parameter, and createTransferPathLeg() passes the already-correct PathBuilderLeg#toStop() through instead of leaving it to be recomputed. This is also one step closer to NOT hold a reference to the transfer in transfer arrivals - this is not needed and would save memory and speed up the search.

The existing basic-path forward/reverse-search test only asserts on the path's rendered string, which (misleadingly) does not depend on TransferPathLeg#toStop() and therefore didn't catch this. Added a direct assertion on the reverse-search transfer leg's fromStop()/toStop() to PathMapperTest, which fails with the expected <3> but was <2> before this fix and passes after it.

Issue

closes #8047

Unit tests

✅ Updated

Documentation

✅ JavaDoc added

Changelog

✅ Minor bugfix - not visible in the APIs

Bumping the serialization version id

🟥 No serialized data changed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.62%. Comparing base (1110116) to head (e0dc038).

Additional details and impacted files

@@            Coverage Diff             @@
##             dev-2.x    #8062   +/-   ##
==========================================
  Coverage      74.61%   74.62%           
- Complexity     22875    22876    +1     
==========================================
  Files           2533     2533           
  Lines          88523    88523           
  Branches        8752     8752           
==========================================
+ Hits           66055    66058    +3     
+ Misses         19399    19396    -3     
  Partials        3069     3069           

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