core: stdcm: fix Offset<Path> conversion when route has previous blocks

[eckter]

Fix #10339 (but only parts of it)

When the first route has several blocks and we don't start on the first one, the Offset<Path> would be off by the previous blocks length. Resulting is mismatched conflicts.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.64%. Comparing base (1004424) to head (a2efb67).
Report is 8 commits behind head on dev.

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Additional details and impacted files

@@            Coverage Diff             @@
##              dev   #10380      +/-   ##
==========================================
- Coverage   81.65%   81.64%   -0.01%     
==========================================
  Files        1066     1066              
  Lines      105687   105692       +5     
  Branches      727      727              
==========================================
+ Hits        86294    86296       +2     
- Misses      19352    19355       +3     
  Partials       41       41              

Flag	Coverage Δ
editoast	73.68% <ø> (-0.01%)	⬇️
front	89.33% <ø> (ø)
gateway	2.18% <ø> (ø)
osrdyne	3.28% <ø> (ø)
railjson_generator	87.50% <ø> (ø)
tests	87.05% <ø> (ø)

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