Parity sprint: verify remaining topics + image-mode CI + perf harness

[iahncajigas]

Summary

• Verified parity branch updates from commit 540519f and extended CI hardening
• Added pinned MATLAB reference checkout (fixed SHA) across parity workflows
• Added canonical validation artifact documentation and stable artifact names
• Added function-level performance parity harness (runtime + memory proxy) with MATLAB baseline fixtures
• Added nightly/PR performance workflow and regression gating

Key additions

• tools/parity/checkout_matlab_reference.py + parity/matlab_reference.yml
• CANONICAL_VALIDATION_ARTIFACTS.md
• tools/performance/run_python_benchmarks.py
• tools/performance/compare_matlab_python_performance.py
• matlab/benchmark/run_matlab_performance_benchmarks.m
• .github/workflows/performance-parity.yml

Validation performed locally

• pytest -q
• Functional parity gate
• Numeric drift gate
• Example output spec gate
• Full gate-mode PDF validation build (all examples)
• Image-mode parity check
• Python benchmark harness + MATLAB/Python performance comparator