core: stdcm: reduce visited margin

[eckter]

The value was too large for takeovers with fast trains, we'd "miss" an opening and let two trains pass instead of one.

This will have a performance cost, but there isn't any way around it. We were likely missing a few solutions in edge cases before.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.88%. Comparing base (ecb261f) to head (be9dc06).
Report is 50 commits behind head on dev.

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Additional details and impacted files

@@            Coverage Diff             @@
##              dev   #10674      +/-   ##
==========================================
+ Coverage   81.85%   81.88%   +0.02%     
==========================================
  Files        1075     1078       +3     
  Lines      107172   107145      -27     
  Branches      728      724       -4     
==========================================
+ Hits        87726    87732       +6     
+ Misses      19407    19373      -34     
- Partials       39       40       +1     

Flag	Coverage Δ
editoast	74.16% <ø> (-0.08%)	⬇️
front	89.45% <ø> (+0.06%)	⬆️
gateway	2.18% <ø> (ø)
osrdyne	3.28% <ø> (ø)
railjson_generator	87.50% <ø> (ø)
tests	88.14% <ø> (ø)

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