test: Strengthen example validation with AST checks.

[nedseb]

Description

The current test_examples.py validates syntax and import consistency but cannot catch runtime errors like undefined names (MENU_BUTTON in dpad_piano.py was a recent example).

Tasks

• Add an AST check to detect obvious undefined global names in examples
• Target lib/*/examples/*.py
• Consider a mode with a fake machine module for import/initialisation validation when the example is compatible
• Integrate into make test-examples and CI

Priority

P0 — prevents broken examples from being merged

Related

• PR examples: Add practical examples for mcp23009e driver. #247 — dpad_piano.py had undefined MENU_BUTTON
• ci: Add example validation to CI workflow. #215 — CI example validation workflow