test(parser): add mixed auth corpus

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Summary

• add a 150-line sanitized mixed syslog auth corpus for dirty-input parser review
• lock parser counts for parsed events, warnings, blank lines, unknown-pattern buckets, and failure categories
• document the mixed corpus in parser coverage notes, parser conformance matrix, and changelog

Validation

• cmake -S . -B build
• cmake --build build
• ctest --test-dir build -C Debug --output-on-failure
• git diff --cached --check
• rg privacy/secret pattern scan on changed public files

Safety

• Fixture uses documentation-range IPs, synthetic hostnames, and synthetic usernames only.
• Unsupported evidence remains warning telemetry and is not counted as detector input.