Replace scroll-to-selection flag with one-shot postLayoutAction executed after layout

- Introduce postLayoutAction closure to run once after the next layout pass.
- Schedule scrollToVisible for the current selection from UITextInput and text
  change paths.
- Execute and clear the action in layoutSubviews.
- Removes needsScrollToSelection, simplifying and aligning with UITextView’s
  _scrollToSelectionIfNeeded behavior.