feat: add outcome-limited split settings

[egillax]

Summary

• add createOutcomeLimitedSplitSettings() for training on a target number of outcome-positive rows
• support exact row-level stratified splitting and approximate subject-level splitting that keeps subjects together
• document the learning-curve rationale with the John et al. citation and add NEWS entry

Subject mode intentionally mirrors existing subjectSplitter() fold assignment behavior for compatibility. More flexible grouped/stratified group splitting can be handled separately.

Closes #396

Tests

• Rscript -e 'Sys.setenv(NOT_CRAN="true"); devtools::test(filter="dataSplitting", reporter="summary")'\n- Rscript -e 'Sys.setenv(NOT_CRAN="true"); devtools::test(filter="paramchecks", reporter="summary")'\n- Rscript -e 'Sys.setenv(NOT_CRAN="true"); devtools::test(reporter="summary")'\n- Rscript -e 'tools::checkRd("man/createOutcomeLimitedSplitSettings.Rd")'\n- R CMD Rdconv --type=txt man/createOutcomeLimitedSplitSettings.Rd\n- git diff --check

[codecov]

Codecov Report

❌ Patch coverage is 99.41860% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 89.89%. Comparing base (b9e7b14) to head (843e617).

Files with missing lines	Patch %	Lines
R/DataSplitting.R	99.39%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #670      +/-   ##
===========================================
+ Coverage    89.77%   89.89%   +0.11%     
===========================================
  Files           49       49              
  Lines        13740    13911     +171     
===========================================
+ Hits         12335    12505     +170     
- Misses        1405     1406       +1     

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