[pcheek1991]

Summary

• Add an include-free, allocation-free C-compatible Pentagonal XOR header.
• Implement the five-input AND, OR, odd-parity XOR probability, partial score, and fifth-root legacy score.
• Improve fifth-root range reduction so very small and large finite magnitudes are handled more reliably than a fixed 20-step Newton iteration from 1.0.
• Add compile-time Windows legacy-target classification using _WIN32 and _WIN32_WINNT; document that this describes the build target, not the runtime host OS.
• Compile and test the header as strict C11 in the existing cross-platform CMake/CTest matrix.
• Update the component documentation and keep the separate C++20 trust-boundary guard distinct.

Validation

The existing workflow should compile C11 and C++20 targets and run the added numeric tests alongside the existing security-guard tests on Linux, Windows, and macOS. This PR has not yet been validated by GitHub Actions; results will be checked after the run starts.

Scope notes

The header expects five finite feature values in [0,1] for the combined score API. The fifth-root helper supports finite signed inputs; NaN and infinities are outside its contract. The Windows macro is compile-time target selection, not reliable runtime OS detection.