refactor: update paxhtml to use bump allocator API

- Update paxhtml dependency to latest version (a5342664) which uses
  bumpalo bump allocator for efficient memory management
- Update all html! macro calls to use new `in bump;` syntax
- Update all function signatures to accept bump allocator reference
- Update Builder, Document, Element, and Attribute usage with new API
- Update syntax highlighter to use bump allocator for Element creation

Author: claude