fix(merge_from_private): fix stop position calculation

[soblin]

Description

In https://github.com/autowarefoundation/autoware.universe/pull/5972/files, the stopline offset confused with baselink2front.

Related links

Tests performed

https://evaluation.tier4.jp/evaluation/reports/ed4aeae1-f94b-580e-81e1-bcfa42680e6f?project_id=prd_jt

before

after

Notes for reviewers

Interface changes

none

Effects on system behavior

none

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[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (cc266ea) 14.38% compared to head (d67d132) 14.38%.
Report is 9 commits behind head on main.

Files	Patch %	Lines
...ction_module/src/scene_merge_from_private_road.cpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6286   +/-   ##
=======================================
  Coverage   14.38%   14.38%           
=======================================
  Files        1907     1907           
  Lines      129943   129943           
  Branches    37616    37616           
=======================================
  Hits        18695    18695           
  Misses      90238    90238           
  Partials    21010    21010           

Flag	Coverage Δ		*Carryforward flag
differential	4.69% <0.00%> (?)
total	14.38% <ø> (+<0.01%)	⬆️	Carriedforward from cc266ea

*This pull request uses carry forward flags. Click here to find out more.

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