refactor(learn): the editor's general parts move to halkyon_editor (ADR 0033, D37)

Todo stores editor documents too, so the document, the editor, its view,
the format bar, links, the clipboard, scripts and EditorBlocks move, with
their strings and tests, to a shared package on the same appflowy_editor
fork. What is Learn's alone stays in Learn on top of it, and what the
editor took from it directly is inverted: EditorBlocks gains textMarks,
formatActions and onEditorClosed, RichContentEditor a wrapEditor, and an
InsertableBlock's name takes the BuildContext. Learn's screens use
LearnContentEditor and LearnContentView, which add Learn's blocks and the
grammar check. Learn behaves as before.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

packages/halkyon_editor/README.md

@@ -0,0 +1,65 @@
+# halkyon_editor
+
+Halkyon's rich text, shared by its apps: the document as an app keeps it, the
+one editor and the one read-only view of it, built on
+[`appflowy_editor`](https://github.com/AppFlowy-IO/appflowy-editor) (our fork,
+pinned, as every app's), with their texts in British English, American English
+and German. Moved out of Halkyon Learn on 6 Nov 2026, when Halkyon Todo took
+editor documents too (ADR 0033, D37).
+
+## Using it
+
+An app installs `HalkyonEditorLocalizations.delegate` beside its other
+delegates, and halkyon_ui's, whose sheets the Insert menu opens in.
+
+```dart
+// Stored: RichContent is the editor's document JSON and its format's version.
+final content = RichContent.plain('Buy milk'); // or RichContent.empty()
+content.plainText; // 'Buy milk'
+content.isBlank;   // false
+final stored = content.toJson();
+RichContent.fromJson(stored);
+
+RichContentEditor(
+  initialContent: content,
+  onChanged: (content) => save(content),
+  formatBar: true,
+);
+RichContentView(content: content);
+```
+
+## What it holds
+
+- **`RichContent`** (`lib/src/content/`): pure Dart, the document in JSON.
+  `content_blocks.dart` names every block's type and attributes across the
+  apps, for `plainText` and for what a block's type is called for good.
+- **`RichContentEditor`, `RichContentView`** (`lib/src/editor/`): the editor,
+  with the `FormatBar` docked below it while it has the focus, copying and
+  pasting that keep the formats (`EditorClipboard`), links (`editLink`, Ctrl+K)
+  and superscript and subscript; the view, display only, taking the height of
+  its content.
+- **`EditorBlocks`**: what an editor knows. `basicBlocks` is what it knows
+  without an app (the editor's own builders; headings, lists, a checklist, a
+  quote, a divider and a table to insert). An app adds its own blocks, and
+  passes them as `blocks:` or through `EditorBlocksScope`, with:
+  - `builders`, `insertable` (the Insert sheet and the desktop's `/` menu show
+    the same list) and `shortcuts`;
+  - `textMarks`, how marks of its own are drawn, each wrapping the one before
+    it, and `formatActions`, their buttons, after subscript;
+  - `onEditorClosed`, for what its text left outside the editor.
+  `plus` joins two sets, `offering` narrows what is offered and nothing else.
+- **`AtomicBlock`, `BlockNotice`**: for an app's block taken whole, selected
+  and removed as one. A type nothing knows shows as needing a newer app, and
+  stays as it is.
+
+`RichContentEditor`'s `wrapEditor` puts what an app adds around the editor
+itself, knowing its state and focus (Learn's check of the spelling).
+
+Editors are tested as in Learn: host one in a scrollable body with a slot of
+fixed height, `pump(const Duration(seconds: 1))` after changes, and find its
+text with `findRichText: true`.
+
+## Licence
+
+GNU Affero General Public License, version 3 or any later version
+(`AGPL-3.0-or-later`), as the other packages of the apps; see [LICENSE](LICENSE).