apply_syntax_to_buffer() was called every frame and unconditionally reset
edit mode from the file extension, making it impossible to toggle out of
writing mode for .txt/.md files. Add edit_mode_detected_ flag to Buffer so
auto-detection runs once per buffer. Writing mode now also disables syntax
highlighting as intended. Propagate edit_mode_ through Buffer copy/move ops.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
