fix(cxx): cppcheck include-path harvesting and information-record classification

[karabelaselias]

Problem

On C++ projects whose sources include headers outside the scanned tree (vendored or sibling-package code), the cppcheck phase produced unactionable findings:

1. _cppcheck_include_args derived -I dirs only from in-scope headers, so external include roots recorded in the project's own compile_commands.json were never passed to cppcheck — every external header emitted missingInclude records.
2. cppcheck's information category (missing include paths, checker reports, normalization notes) is non-actionable by definition but was captured as findings.

Fix

• Harvest -I paths from compile_commands.json entries matching the scanned files, falling back gracefully when the database is absent or malformed.
• Classify information-severity records as non-actionable in the phase filter.

Verification

• New unit tests: information-record classifier, compile-db harvesting, malformed-database handling.
• Full upstream suite: 5818 passed, 3 skipped.
• Real-world check: C++17 rod-solver repo (114 files) — 47 information-level records and recurring missingInclude noise dissolved after the fix.

[awdemos]

The core fix is sound and I verified the important part end-to-end: cppcheck information: records keep their severity prefix through parse_gnu (the parser only strips file:line), so _is_information_record is live production code — both information records dropped, real error/warning findings kept, and the header-suffix guard correctly spares a .cpp "is never used" entry. Malformed/absent compile_commands.json is handled gracefully (+3 tests pass).

Four things to address before merging:

1. The body is inaccurate: it claims _cppcheck_include_args already existed and "derived -I dirs only from in-scope headers" — on main that function does not exist and cppcheck gets zero -I args. The function is new in this PR. Please correct the description.
2. Unadvertised flag changes: --check-level=exhaustive requires cppcheck ≥2.16 — older distro cppcheck rejects it, so every batch errors and the detector yields zero findings (only reduced-coverage metadata) for those users. Also --language=c++ --std=c++17 is forced on .c files, and --inline-suppr is added. None of these are mentioned in the body; each needs justification or removal.
3. Unadvertised _is_header_unused_diagnostic filter — also swallows unusedFunction "… is never used." findings located in headers. Over-suppression; at minimum disclose it.
4. New ruff I001 import-sort error (likely red in CI). Nit: relative -I paths in the compile db are resolved without the entry's directory field.