feat(fleather): add iPad Scribble support

[zhpoo]

Changes:

• expose stylusHandwritingEnabled from FleatherEditor, FleatherField, and RawEditor;
• implement Flutter's ScribbleClient lifecycle for editable iOS editors;
• report UTF-16 character, caret, composing, size, and transform geometry to the text input connection;
• render and remove Scribble placeholders without changing the Parchment document;
• support selection boxes across Fleather block nodes;
• accept Scribble handwriting across the full editor surface without moving the current selection on pen-down;
• connect Fleather history to the iOS system undo manager for Pencil toolbar undo and redo;
• reject Scribble input whose center is outside the editor bounds;
• keep Scribble disabled at runtime on non-iOS platforms and in read-only editors.

[codecov]

Codecov Report

❌ Patch coverage is 93.58491% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.23%. Comparing base (1ee8bdc) to head (10aeb47).

Files with missing lines	Patch %	Lines
packages/fleather/lib/src/widgets/editor.dart	92.53%	10 Missing ⚠️
packages/fleather/lib/src/widgets/text_line.dart	84.21%	6 Missing ⚠️
packages/fleather/lib/src/widgets/history.dart	97.95%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #586      +/-   ##
==========================================
+ Coverage   88.11%   88.23%   +0.12%     
==========================================
  Files          63       63              
  Lines       10899    11156     +257     
==========================================
+ Hits         9604     9844     +240     
- Misses       1295     1312      +17     

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