Add secondary_cursors field to Editor with insert_char_multi, delete_back_multi, delete_forward_multi methods. Right-to-left processing ensures position shifts don't corrupt earlier insertions. 7 new tests: add/clear, all_positions, insert, delete_back, delete_forward, unicode, duplicate-add.
- Add x11proto to redbear-full.toml package list - libxau recipe updated with x11proto dependency and custom build script - Fixes libxau build failure: 'Package xproto was not found'