- append/smart_merge now write headers on empty sheet (write_headers=True)
- smart_merge: replace append_values with insertDimension(inheritFromBefore=False)
  + batch_update_values to prevent header formatting leaking to data rows
- Add table_start parameter (default "A1") to append and smart_merge modes
- Fix: smart_merge on empty sheet inserted data before header (insert_start off-by-one)
- Fix: append empty-check now reads single cell instead of whole row