core: split margins at any scheduled point

[eckter]

Margin ranges need to be split at scheduled points for consistency with what the front-end expects.

May fix #8833

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 36.92%. Comparing base (99094af) to head (40672ec).

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Additional details and impacted files

@@            Coverage Diff            @@
##                dev    #9224   +/-   ##
=========================================
  Coverage     36.91%   36.92%           
  Complexity     2242     2242           
=========================================
  Files          1255     1255           
  Lines        116958   116972   +14     
  Branches       3270     3272    +2     
=========================================
+ Hits          43178    43189   +11     
- Misses        71835    71838    +3     
  Partials       1945     1945           

Flag	Coverage Δ
core	74.80% <100.00%> (+0.02%)	⬆️
editoast	72.26% <ø> (-0.02%)	⬇️
front	15.61% <ø> (ø)
gateway	2.22% <ø> (ø)
osrdyne	2.46% <ø> (ø)
railjson_generator	87.49% <ø> (ø)
tests	86.71% <ø> (ø)

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