test(parser): add noisy auth fixture matrix

[stacknil]

Summary

• add a sanitized noisy syslog auth sample with malformed timestamps, unsupported sshd lines, partial PAM lines, sudo denial variants, blank/irrelevant lines, mixed hostnames, and unusual synthetic usernames
• add a locked parser-matrix expected JSON for parser quality fields and warning visibility
• add parser/report tests proving parsed/unparsed counts, stable top_unknown_patterns, report.json parse_success_rate, and zero findings from unsupported lines
• document how reviewers should read noisy parser coverage in docs/parser-coverage-notes.md

Validation

• cmake --build build --config Debug --target test_parser
• cmake --build build --config Debug --target test_report
• ctest --test-dir build -C Debug -R "parser|report" --output-on-failure
• cmake --build build --config Debug
• ctest --test-dir build -C Debug --output-on-failure
• loglens --mode syslog --year 2026 assets/noisy_auth_sample.log build/noisy_auth_report_check
• git diff --check main...HEAD
• diff scans for local paths, secret assignment patterns, private-key markers, and non-documentation IP addresses

Safety

• Fixture data uses synthetic hostnames/usernames and 203.0.113.x documentation addresses.
• Unsupported lines remain parser warnings/telemetry and do not become detector findings.
• No parser behavior or detector mapping changes.