fix(pdf-accessibility): include Matterhorn 15-003 Headers/ID exception in TH scope rules

[AdamMagued]

Closes #201

Why This Change is Needed

In the PDF accessibility rules (PDFUA.19.002 and PDFBP.TAB.SCOPE_SET), rule definitions previously required all TH cells to explicitly declare a /Scope attribute and flagged violations as errors, without accounting for the canonical exception specified in Matterhorn Protocol 1.1 failure condition 15-003:

"15-003: In a table not organized with Headers attributes and IDs, a TH cell does not contain a Scope attribute."

Crucially, tables that associate cells using Headers attributes and ID values satisfy the requirement without /Scope attributes. Furthermore, condition 15-003 is designated in the protocol as requiring human judgement rather than automated machine checking. Flagging missing /Scope as an automated error produced false positives and conflicted with the protocol designation.

What Changes Were Made

• Changed PDFUA.19.002 severity from error to warning to reflect that condition 15-003 requires human judgement.
• Updated PDFUA.19.002 rule description in skills/pdf-accessibility/references/rule-layers.md to specify the exception: "TH cell has neither a /Scope attribute nor Headers/ID associations".
• Updated PDFUA.19.002 rule description in skills/pdf-scan-config/references/complete-rule-reference.md to "TH cells have Scope or Headers/ID associations".
• Updated PDFBP.TAB.SCOPE_SET rule descriptions in both references to note the Headers and IDs alternative.
• Added explicit notes to rule-layers.md and complete-rule-reference.md explaining that tables organized through Headers and IDs satisfy Matterhorn 15-003 without /Scope attributes.
• Updated skills/pdf-accessibility/references/remediation-guidance.md under Table Remediation to mention header-ID associations per Matterhorn 15-003 and added an explanatory note.

Verification

• Executed npm run verify in the sandbox container; all 15 gates passed cleanly.
• Verified agent skills specification, package rules, frontmatter conformance, context budgets, findings contract, report renderer, enforcement gate, MCP server, markdown accessibility, markdown linter, version consistency, dispatch matrices, generated documentation, and dependency advisories.