Align PDFUA rule references with Matterhorn Protocol 1.1 checkpoints

[AdamMagued]

Summary

Resolves #202 by aligning PDFUA rule references with Matterhorn Protocol 1.1 checkpoints and adding explicit Checkpoint Topic columns.

Context

In pdf-accessibility and pdf-scan-config, several rule references previously cited checkpoint numbers that did not match the official Matterhorn Protocol 1.1 checkpoint organization:

• Language rules (PDFUA.06.001, .002, .003) were mapped under checkpoint 06 (Metadata), whereas Language is checkpoint 11 (Natural Language).
• Heading skipping (PDFUA.07.001) and heading tagging (PDFUA.21.001) were mapped under checkpoints 07 and 21, whereas Headings is checkpoint 14.
• Inline figure tagging (PDFUA.14.001) was mapped under checkpoint 14, whereas Graphics is checkpoint 13.
• Formula tagging (PDFUA.15.001) was mapped under checkpoint 15, whereas Math is checkpoint 17.
• Table rules (PDFUA.19.001, .002, .003) were mapped under checkpoint 19, whereas Tables is checkpoint 15.
• List tagging (PDFUA.20.001) was mapped under checkpoint 20, whereas Lists is checkpoint 16.
• Form field rules (PDFUA.26.001, .002, .003) were mapped under checkpoint 26, whereas Non-Interactive Forms is checkpoint 24.
• Artifacts in structure tree (PDFUA.17.001) was mapped under checkpoint 17, whereas Real Content / Artifacts is checkpoint 01.
• Tab order consistency (PDFUA.25.001) was mapped under checkpoint 25, whereas Annotations tab order is checkpoint 28.
• XMP/Info consistency (PDFUA.30.001) and PDF/UA identifier (PDFUA.31.001) were mapped under 30 and 31, whereas Metadata is checkpoint 06.

Changes

1. Updated skills/pdf-accessibility/references/rule-layers.md:
   • Clarified introductory description that rule IDs serve as stable identifiers while the Matterhorn Checkpoint and Checkpoint Topic columns indicate the corresponding Matterhorn Protocol 1.1 checkpoint.
   • Updated the table header to | ID | Matterhorn Checkpoint | Checkpoint Topic | Severity | Description |.
   • Corrected checkpoint numbers and added official topic names across all 30 Layer 1 rules.
2. Updated skills/pdf-scan-config/references/complete-rule-reference.md:
   • Updated introductory description and table headers to | ID | Matterhorn Checkpoint | Checkpoint Topic | Severity | What It Checks |.
   • Corrected checkpoint numbers and added official topic names to maintain complete consistency across reference documentation.

Verification

• Ran full skill validation: node scripts/validate-skills.mjs (108 skills passed).
• Ran conformance check: node scripts/check-skill-conformance.mjs (108 skills conformant).
• Ran context budgets: node skills/a11y-core/scripts/measure-context.mjs --check budgets.json (OK).
• Ran findings contract test: node skills/a11y-core/scripts/test-findings-contract.mjs (88 passed, 0 failed).
• Ran report renderer self-test: node skills/a11y-core/scripts/render-report.mjs --self-test (OK).
• Ran markdown accessibility lint: node .github/scripts/markdown-a11y-lint.mjs . --fail-on error (0 issues).
• Ran spec reference validator: node scripts/verify-spec.mjs (108 skills conformant).
• Ran table description check: node scripts/describe-tables.mjs (0 changes needed).
• Ran release version consistency: node scripts/check-release-consistency.js (consistent).