fix(rules): align pdfua rule checkpoint numbers with matterhorn protocol

[AdamMagued]

Resolves #202

Why This Change is Needed

In the PDF accessibility rule references (skills/pdf-accessibility/references/rule-layers.md and skills/pdf-scan-config/references/complete-rule-reference.md), the Checkpoint column previously duplicated the numeric middle segment of each rule ID (e.g. citing checkpoint 19 for table rules, 06 for language rules, 07 for headings).

As detailed in #202, Matterhorn Protocol 1.1 defines 31 canonical checkpoints with specific subjects:

• Checkpoint 06 is Metadata; language rules (/Lang, BCP 47) belong to Checkpoint 11 (Declared Natural Language).
• Checkpoint 07 is Dictionary; heading rules belong to Checkpoint 14 (Headings).
• Checkpoint 14 is Headings; figure/inline image rules belong to Checkpoint 13 (Graphics).
• Checkpoint 15 is Tables; formula rules belong to Checkpoint 17 (Mathematical Expressions).
• Checkpoint 17 is Mathematical Expressions; artifact rules belong to Checkpoint 01 (Real content tagged).
• Checkpoint 19 is Notes and References; table rules belong to Checkpoint 15 (Tables).
• Checkpoint 20 is Optional Content; list rules belong to Checkpoint 16 (Lists).
• Checkpoint 21 is Embedded Files; heading rules belong to Checkpoint 14 (Headings).
• Checkpoint 25 is XFA; tab order rules belong to Checkpoint 27 (Navigation).
• Checkpoint 26 is Security; form field rules belong to Checkpoint 24 (Non-Interactive Forms).
• Checkpoints 30 (XObjects) and 31 (Fonts) do not cover metadata; XMP/Info consistency and PDF/UA identification belong to Checkpoint 06 (Metadata).

What Changes Were Made

1. In skills/pdf-accessibility/references/rule-layers.md:
   • Updated the Checkpoint column for all 30 PDF/UA conformance rules to reference their canonical Matterhorn Protocol 1.1 checkpoint numbers.
   • Updated the introductory description to specify Matterhorn Protocol 1.1.
2. In skills/pdf-scan-config/references/complete-rule-reference.md:
   • Added and populated the Checkpoint column for all Layer 1 PDFUA rules aligned with Matterhorn Protocol 1.1.
   • Updated the introductory description to reference Matterhorn Protocol 1.1.

Verification

• npm run verify:skills (validated 108 skills)
• npm run verify:conformance (all frontmatter conformant)
• npm run verify:markdown (0 markdown accessibility errors)
• npm run verify:budgets (all context budgets OK)
• npm run verify:findings (all findings contract tests passed)
• npm run verify:report (render-report self-test passed)
• npm run build:matrices (matrices verified current)

[AdamMagued]

closing in favor of #216 which includes the complete checkpoint alignment table. apologies for opening two separate PRs on this!