[Fix] Restore future plan in multicore utility

[antoniojbt]

Summary

• Preserve any existing future plan before configuring parallel execution
• Reset to sequential and reinstate previous plan on exit to terminate workers cleanly

Testing

• Rscript -e "styler::style_pkg()"
• Rscript -e "lintr::lint_package()" (warnings)
• Rscript -e "devtools::test(reporter = 'summary')"
• Rscript -e "covr::report()" (no output)
• Rscript -e "devtools::check(manual = FALSE)" (warning: qpdf missing; notes: compare, doFuture unavailable)

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https://chatgpt.com/codex/tasks/task_e_68b2059784e0832696b8ffc745a1eeb5

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.01%. Comparing base (e412fcb) to head (7f10eeb).
⚠️ Report is 11 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #146      +/-   ##
==========================================
+ Coverage   83.97%   84.01%   +0.03%     
==========================================
  Files          64       64              
  Lines        1423     1426       +3     
==========================================
+ Hits         1195     1198       +3     
  Misses        228      228              

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