Fix C++ static test coverage mapping

[AustenMan]

Summary

• map exercised public headers to a unique same-stem C/C++ implementation while retaining ambiguity and unused-include guards
• recognize Catch2 tests/assertions and Qt CMake target commands
• remove repeated filesystem path resolution and replace the catastrophic C++ testability regex with bounded lexical checks

Verification

• python -m ruff check desloppify/languages/cxx/test_coverage.py desloppify/languages/cxx/tests/test_coverage.py
• python -m pytest desloppify/languages/cxx/tests -q (53 passed)
• read-only calibration against dicom_viewer: 478-file graph completes; representative beam_geometry.cpp, rt_plan_reader.cpp, and rt_dose_command_service.cpp false gaps are removed

The broader shared coverage suite currently has unrelated Windows path-normalization failures in existing Python/TypeScript tests; this change does not touch those paths.