Controles bilingües / bilingual quizzes

Canonical bank (packaged for Colab/PyPI):
  src/didactichem/data/controles/es
  src/didactichem/data/controles/en

Do NOT put YAML under notebooks/controles/ — share notebooks only;
runtime loads banks from the installed didactichem package.

Regenerate BOTH languages:
  python scripts/generar_controles.py

Student notebooks use didactichem.controles_dir() (package data).
