[codex] Add render parity rule pack

[PSkinnerTech]

Summary

• Adds a pure render parity helper that compares raw and rendered SEO/GEO-critical signals only when rendered evidence exists.
• Registers render parity rule metadata and emits findings for changed or removed title, description, canonical, missing rendered H1/content, structured-data loss, and broad raw/rendered text mismatch.
• Adds helper, rule-engine, audit-level regression coverage plus validation and changelog updates.

Why

Rendered audits previously collected rendered evidence, but the rule engine only had limited raw/rendered mismatch behavior. This PR turns explicit rendered evidence into deterministic findings without changing default rendering behavior or making ranking claims.

Validation

• node --test packages/cli/test/render-parity.test.mjs packages/cli/test/rules.test.mjs packages/cli/test/rule-engine.test.mjs packages/cli/test/audit.test.mjs -> 39 passed
• npm test -> 198 passed
• npm run validate -> ok, requiredFiles 97, rules 39
• git diff --check -> clean