Add some tests

[polyedr]

This PR adds two tiny, fast test files:

• repo_health: ensures README.md exists and compiles all tracked .py files to catch syntax errors.
• markdown_snippets: parses Python code fences (python … ) in all .md files via ast.parse (limited to short blocks).

These tests help keep teaching materials consistent and syntax-correct without running any exercises.
No functional changes.

To run locally:

pip install pytest
python -m pytest -q