Removes the unreliable viewport-centre cursor heuristic. Switching to
edit mode now always positions the cursor at text.length and scrolls to
the bottom, whether arriving from a regular tap or a long-press. The
long-press scroll-to-end on initial load (view mode) is unchanged.
Cleans up _cursorAtEnd flag, _editorKey / GlobalKey, _getOffsetAtViewportCenter,
and the public WrappedCodeFieldState / findRenderEditable that were only
needed for the removed feature.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>