[codex] enforce parity category thresholds

[andrewtdiz]

Linked issues

Closes #794.

Summary

Adds a dedicated parity threshold gate for test-parity/reports/latest.json that preserves the existing global min_parity_pct check and adds per-category/module threshold enforcement.

The new gate groups report entries into stable categories such as parity/http, node-suite/url, and legacy/issue, writes JSON/markdown summaries, and fails CI when any configured or default category drops below its threshold.

Why this batch

This is a single CI/reporting cut: script, threshold schema, and workflow wiring need to land together so the existing parity job can produce and enforce the new report artifact.

Tests added / validation

• python3 -m py_compile scripts/parity_threshold_gate.py
• jq empty test-parity/threshold.json
• synthetic passing report: configured known-gap category below 100% is accepted
• synthetic failing report: unconfigured node-suite/url compile failure is rejected by the default 100% category gate
• git diff --check
• ./scripts/check_file_size.sh

Known limitations

The initial category exceptions mirror current known broad inventory gaps rather than raising those categories. Future parity improvements can tighten the corresponding min_parity_pct entries as gaps are retired.

Non-goals

• No runtime parity behavior changes.
• No changes to known_failures.json semantics.
• No changes to the parity runner test selection.