[codex] Add three-parameter double logistic selectivity

[jimianelli]

Summary

• Add a DoubleLogistic3Selectivity C++ functor implementing the 3-parameter double logistic form from jimianelli/selex.
• Expose the new selectivity through the Rcpp module interface, R exports, and default parameter creation as DoubleLogistic3.
• Add focused gtest and testthat coverage for the math helper, time-indexed evaluation, and R interface.

Validation

• ./build/tests/gtest/fims_math_double_logistic3
• ./build/tests/gtest/DoubleLogistic3_DoubleLogistic3Selectivity_Evaluate
• R CMD INSTALL .
• Rscript -e "library(FIMS); testthat::test_file('tests/testthat/test-rcpp-selectivity.R')"

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Checklist

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[github-actions]

🎨 Chore: code formatting workflow

Our automated workflows cannot run on forks because of permission issues, and thus, we ask that you run the following code locally and push any changes that are created to your feature branch. You will only be reminded of this once per PR. Thank you!

Format C++ code

1. Install clang-format version 18.0.0
2. Run the following command from the repository root:

   clang-format -i --style="{BasedOnStyle: Google, SortIncludes: false}" $(find ./inst/include ./src ./tests/gtest -name "*.hpp" -o -name "*.cpp")

Format R code

1. Install {styler} and {roxygen2}
2. Run the following commands in R from the repository root:

styler::style_pkg() # Style R code
roxygen2::roxygenise() # Update documentation
styler::style_pkg() # Style R code again
roxygen2::roxygenise() # Update documentation again
usethis::use_tidy_description() # Style DESCRIPTION file

Push changes

1. Commit the formatting with a commit message of "Chore: format feature branch"
2. Push to your fork

[codecov]

Codecov Report

❌ Patch coverage is 41.29353% with 118 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.40%. Comparing base (b9d8f56) to head (d021fc7).

Files with missing lines	Patch %	Lines
...e/interface/rcpp/rcpp_objects/rcpp_selectivity.hpp	16.40%	107 Missing ⚠️
R/create_default_parameters.R	20.00%	8 Missing ⚠️
inst/include/common/fims_math.hpp	82.35%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1436      +/-   ##
==========================================
+ Coverage   82.97%   88.40%   +5.42%     
==========================================
  Files          54       85      +31     
  Lines        2214     8613    +6399     
  Branches      578      581       +3     
==========================================
+ Hits         1837     7614    +5777     
- Misses        277      965     +688     
+ Partials      100       34      -66     

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