T21a: Documents, editing

- markdown read with pulldown-cmark: marks for live styling, blocks, links, words
- forms::Highlight gains weight, slant and strike; measure and paint share runs
- Inter Italic embedded, so emphasis reads slanted
- forms::mention's popup and keys become Suggestions, shared with the editors
- RichTextEditor with FixedFormatToolbar, the floating toolbar, LinkEditor,
  @ and : suggestions, WordCount and ReadingTime
- MarkdownEditor: visual (a pressed block opens as source), source, split
- BlockEditor: prose, lists, to-dos, quotes, callouts, toggles, code, math,
  diagrams the app draws, dividers, tables that grow and merge, pictures to
  resize and align, cards for video and pages, columns, synced text; slash menu,
  markdown shortcuts, handles that drag and open a menu, one undo for all
- TodoItem and Checklist
- icons: heading-1..3, workflow, merge
- gallery: the Documents page with scripted states

TASKS.md

@@ -68,7 +68,7 @@ Per-component lines live in `tasks/`. Tags there:
 - [x] T18 Terminal
 - [x] T19 Git
 - [x] T20 Debug
-- [ ] T21a Documents · Editing — `tasks/ch21-30.md`
+- [x] T21a Documents · Editing — `tasks/ch21-30.md`
 - [ ] T21b Documents · Reading
 - [ ] T21c Documents · Knowledge
 - [ ] T22 Collaboration
@@ -247,3 +247,4 @@ Per-component lines live in `tasks/`. Tags there:
 | T19 | 3 | PASS | |
 | T20 | 1 | FAIL | the demo picked past memory's end; an empty gutter had no height to press; a breakpoint's box also opened it; zoomed ticks lost their places; docs promised bound keys and batch actions |
 | T20 | 2 | PASS | |
+| T21a | 1 | — | pending review |