Add `pdfstandard` metadata variable for specifying PDF standards
(PDF/A, PDF/X, PDF/UA) in LaTeX output. Uses LaTeX's \DocumentMetadata
command which requires LuaLaTeX.
- PDF version requirements are automatically inferred, but can be explicitly
overridden.
- Automatic tagging for standards that require it (ua-1, ua-2, a-2a, a-3a).
- Warning for unsupported standards
- Documentation in MANUAL.txt (Variables for LaTeX, Accessible PDFs)
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>
