[R] Add automated tests and lint pass for R utility functions

[Amasus]

The R analysis toolkit has no automated logic tests — the smoke test only verifies packages load. The utility functions in `result_analysis/utility_functions/` have untested logic and have never been run through `lintr` or `styler`. This issue tracks bringing the R code to the same quality bar as the Python solver.

Sub-issues

• Lint and style pass on utility functions
• Set up testthat infrastructure and `run.py r_unit_test`
• Test flag and utility functions
• Test config loading and descriptor functions
• Test result data transformation functions
• Clean up and test regression functions
• Add R lint check to `run.py`