v1.1.0

• Opt in with tagged: true. Documents are tagged only when created with
  Prawn::Document.new(tagged: true), matching the tagged? query and standard
  "tagged PDF" terminology. The marked: option is removed (no alias).
• Reduced the patching surface: no pdf-core patches. MarkedContent and
  StructureTree now live under Prawn::Accessibility and use only the public
  renderer API; the structure tree is owned by the document and finalized via
  the existing before_render hook. The high-level API is mixed into
  Prawn::Document with include.
• Untagged output is unchanged (byte-for-byte stock Prawn). Header cells are
  flagged at construction, so Cell#header? is consistent before and after draw.
• Removed the low-level @api private symbols renderer.marked?,
  renderer.structure_tree, PDF::Core::MarkedContent,
  PDF::Core::StructureTree, and PDF::Core::ObjectStore#marked?.