test: lock down golden report contracts with regression fixtures

[stacknil]

Closes #12

Summary

• add sanitized end-to-end fixture inputs for report contract coverage
• check in golden report.md and report.json outputs for both syslog_legacy and journalctl_short_full
• add regression tests that compare stable user-visible contract fields rather than incidental formatting noise

Scope

This PR is limited to report-contract regression coverage.
It does not expand parser features, detector behavior, workflows, or local tooling.

Why not whole-file byte equality

Whole-file byte equality would make the suite brittle on blank lines, table separators, JSON indentation, and other presentation-only churn.
The new test locks stable user-visible contract lines and fields so review catches real regressions without turning formatting noise into maintenance overhead.

What is locked down

• summary contract fields in Markdown and JSON
• finding rows / finding objects
• event count tables / entries
• parser quality counts and unknown-pattern buckets
• warning objects and warning rows

What stays flexible

• Markdown blank lines and table separator formatting
• JSON indentation / brace-only noise
• non-contract presentation details that do not change the selected user-visible fields

Verification

• configured with CMake
• built successfully
• ctest --output-on-failure passes, including the new report_contracts target