test(parser): add mixed coverage artifact

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Summary

• add assets/mixed_auth_parser_coverage.json as a reviewer-facing parser observability artifact for the mixed auth corpus
• lock the artifact against live parser output in tests/test_parser.cpp
• link the artifact from parser coverage notes, parser contract, conformance matrix, reviewer path, and changelog

Validation

• cmake -S . -B build
• cmake --build build
• ctest --test-dir build -C Debug --output-on-failure
• git diff --check
• git diff --cached --check
• ConvertFrom-Json validation for assets/mixed_auth_parser_coverage.json
• privacy/secret pattern scan on changed public files

Safety

• Artifact is deterministic and contains only sanitized fixture metadata, documentation-range IPs, synthetic hostnames/usernames, parser counters, event-type counts, and parser warning line references.