Add high-level accessibility API for tagged PDF generation

[craigmcnamara]

Summary

Adds Prawn::Accessibility module providing a user-friendly API for generating Section 508 compliant tagged PDFs.

New document options:

• marked: true — enables tagged PDF mode (structure tree, marked content)
• language: 'en-US' — sets /Lang on the Catalog

New methods on Prawn::Document:

• tagged? — check if document is in tagged mode
• structure(tag, attributes, &block) — wrap content in a structure element with BDC/EMC
• structure_container(tag, attributes, &block) — container element for nesting (children tag themselves)
• artifact(type:, &block) — mark decorative content (footers, borders) excluded from screen readers
• heading(level, text, options) — H1-H6 convenience
• paragraph(text, options, &block) — <P> convenience
• figure(alt_text:, &block) — <Figure> with /Alt

Supported attributes: :Alt, :ActualText, :Lang, :Scope

pdf = Prawn::Document.new(marked: true, language: 'en-US')
pdf.heading(1, 'Document Title')
pdf.paragraph('Body text.')
pdf.structure(:Span, ActualText: 'required') { pdf.text('*') }
pdf.artifact(type: :Pagination) { pdf.text('Page 1') }

Dependency: This PR depends on prawnpdf/pdf-core#67 being merged first. The new accessibility specs require the marked: option and StructureTree class added in that PR. Test failures in CI for release and edge matrix variants are expected until pdf-core#67 is merged and available.

See also companion PR for prawn-table#164.

Addresses prawnpdf/prawn-table#78

Test plan

• 18 new specs covering tagged mode, structure elements, artifacts, headings, paragraphs, figures, ActualText, and full document round-trip
• All 912 specs pass locally (896 existing + 18 new, using local pdf-core fork)
• RuboCop passes
• CI: release and edge tests will pass once pdf-core#67 is merged
• Generate a tagged PDF and verify structure in Adobe Acrobat Tags panel
• Run PAC 2024 accessibility checker

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