[codex] add parity matrix trend gate

[andrewtdiz]

Linked issues

• Closes part of compat: wire the 42-module parity matrix into CI with per-module trend #812

Behavior cluster summary

This adds the CI/reporting slice for the Node parity matrix:

• run_parity_tests.sh now emits compact per-test results records in latest.json.
• scripts/parity_matrix_trend.py builds a per-module matrix for top-level test_parity_<module> runs, including status, known-failure state, Node/Perry line counts, and unified-diff changed-line counts.
• test-parity/parity_matrix_baseline.json seeds the current module-inventory gaps while treating any absent module as must-pass with zero diff lines.
• The existing parity CI job now checks the matrix, writes JSON/Markdown artifacts, and appends the table to the Actions step summary.

Why these changes are batched

The report gate needs all three pieces to be useful: the runner has to expose per-test status, the checker has to convert that into a module trend, and CI has to publish/fail on the artifact. Keeping them together avoids a dead script or an unused report field.

Tests and checks

• python3 -m unittest tests.test_parity_matrix_trend
• bash -n run_parity_tests.sh
• python3 -m py_compile scripts/parity_matrix_trend.py tests/test_parity_matrix_trend.py
• jq empty test-parity/parity_matrix_baseline.json
• local YAML parse of .github/workflows/test.yml
• ./scripts/check_file_size.sh
• cargo fmt --all -- --check
• git diff --check

Known limitations

• I did not run the full parity suite locally because an unrelated cargo build --release -p perry -p perry-runtime -p perry-stdlib job was already active in the shared workspace; starting another full parity run would have kicked off a competing release build.
• Existing module-inventory entries use max_diff_lines: null until a full CI/release parity run records exact diff baselines. New untriaged module failures still fail immediately.

Non-goals

• Importing upstream Node core tests (compat: Node core test subset comparison job #800).
• Changing Node module runtime behavior.
• Editing existing known_failures.json module-inventory rows.