feat(richtext): spelling underlines, tap-to-fix, and native text-checking engine

Own-drawn dotted spelling / grammar / correction underlines with
tap-to-fix guesses and autocorrect-revert, selection-driven checking
(no full-document scan), table-cell underline overlay, and the native
UITextCheckingController engine. Also inverts the composer flag to
forceNewTextInput (Force Text Field v2).

Squashed from the feature/richtext-spell-check and
feature/richtext-native-textchecking topic branches.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>