Add local setup and reproducibility guide

[bofandra]

Adds an optional local workflow alongside the default Google Colab path.

Changes:

• add LOCAL_SETUP.md documenting the Python 3.12 reference environment;
• document clone, optional virtual environment, validator, Module 12 example, notebook-local options, and ignored local artifacts;
• link the guide from README and CONTRIBUTING;
• record it in CHANGELOG;
• extend validation so reproducibility guidance remains present.

No third-party dependency is introduced.