core: stdcm: add fixed time points at start+end of engineering allowances

[eckter]

Fix what I believe to be the last cases of #10412

Though while investigating I found this, when printing the engineering allowances on the final path:

...
margin from 234,337m to 444,837m
margin from 234,337m to 446,622m
margin from 116,065m to 449,152m
margin from 234,337m to 451,259m
...

Meaning: a margin went further back than the previous margin. It doesn't seem to cause any significant issue yet, but it feels very wrong and worth looking into.

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.80%. Comparing base (60a43d5) to head (e589498).
Report is 11 commits behind head on dev.

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Additional details and impacted files

@@            Coverage Diff             @@
##              dev   #10488      +/-   ##
==========================================
+ Coverage   81.78%   81.80%   +0.01%     
==========================================
  Files        1073     1073              
  Lines      106578   106584       +6     
  Branches      731      731              
==========================================
+ Hits        87164    87188      +24     
+ Misses      19375    19357      -18     
  Partials       39       39              

Flag	Coverage Δ
editoast	74.28% <ø> (+0.06%)	⬆️
front	89.34% <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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[Erashin]

Distance bug is probably caused by a length computation bug, which was never really apparent until now. As discussed, it's fine to merge this rn, and you'll look into it later on.