Replace getopt_long with popl.hpp for CLI option parsing (#302)

* Replace getopt_long with popl.hpp for CLI option parsing

- Vendor popl.hpp as a single-header dependency
- Replace manual parsing logic with popl::OptionParser
- Preserve all existing CLI behavior and defaults
- Maintain repeatable options, --pid and -- prog modes
- All 14 existing tests pass (ctest 14/14)

Author: iareARiES