Lists and checkboxes !14

Phase 2, plan D2.

• org-list.el's structure, repairs and writer ported onto EmacsBuffer; insert item (with checkbox), indent/outdent item and tree, move item, toggle checkbox, checkbox statistics cookies.
• Keys in both presets, with when = "item" so M-RET and the M-arrows act on items in lists and on headings elsewhere.
• Oracle matches Emacs on 13 variants at every caret and on list lines in notes, work-notes and cleberg.net content. Fixes found on the way: Emacs-style ^ at end of buffer, match-data tracking for stale structures, forced move-to-column in tabs, items in comment blocks, indent-tabs-mode default in the oracle.
• EmacsBuffer diffs only the touched span: structure and list commands take 1–8 ms on a 1.5 MB file.

233 tests pass.