fix: improve cxx detector scoping

[Dragoy]

Summary

• teach C/C++ test coverage to treat CMake target source membership as direct test coverage
• filter C/C++ security findings to the scoped first-party file set and disable the unsound generic unused-import phase for C++
• add regression tests covering CMake coverage mapping, external-header security filtering, and C++ phase configuration

Test Plan

• python -m pytest desloppify/languages/cxx/tests/test_init.py desloppify/tests/lang/common/test_treesitter_analysis_direct.py desloppify/languages/cxx/tests/test_coverage.py desloppify/languages/cxx/tests/test_security.py -q

[peteromallet]

Review

Three focused improvements to the C++ plugin — all addressing real problems. The implementation is sound.

Looks good

• Security scoping: filtering findings to the scoped first-party file set is the right approach. Empty entry["file"] resolves to CWD which safely gets filtered out.
• CMake coverage mapping: the _CMAKE_SOURCE_SPEC_RE regex handles common patterns well, and the test-tree ancestor walk is correctly scoped.
• Disabling unused-imports for C++ — correct call, #include semantics don't map to the generic tree-sitter unused-import phase.
• _extract_import_name refactor: the new ordering (strip path separators → strip extensions → split namespace separators) correctly handles cases like vendor/json.hpp.

Issues to fix

1. Duplicate test — test_detect_cxx_security_ignores_findings_outside_scanned_files (~line 93 of test_security.py) and test_detect_cxx_security_ignores_findings_outside_scoped_files (~line 613) are nearly identical. Remove one.

2. Missing newline at EOF in test_init.py and test_security.py — will likely fail linting.

Minor suggestions

3. Extra blank lines added between functions in test_coverage.py and mapping_imports.py create noise — the codebase typically uses standard PEP 8 spacing.

4. _discover_additional_test_mapping_files is exported in mapping_imports.__all__ with a leading underscore — slightly unusual for a public export. Since it's called cross-module, consider dropping the underscore.

5. No test for _parse_cmake_source_specs with target_sources() or CMake comments — would be good to add.

Solid PR overall. Fix the duplicate test and EOF issues and it's good to merge.

[peteromallet]

Merged onto the 0.9.9 branch (7fe01b4) with minor cleanup — removed a duplicate test and added missing EOF newlines. Thanks @Dragoy!