Add unit tests for compute_layout() and print_human()

[carpalsgrabby]

Summary

The project currently has no automated tests. Since the core logic lives in compute_layout() and print_human(), adding a small test suite would increase confidence that the layout calculations remain correct as the script evolves.

Tasks

• Add a tests/ folder using pytest.
• Test compute_layout() for:
  • basic cases (small leaves/arity)
  • edge cases (leaves=1, arity=2, large values)
  • padding correctness
  • height calculation logic
• Test print_human() by capturing stdout and verifying formatting.
• Add a simple GitHub Actions workflow to run tests on push/PR.

Benefit

Ensures core logic remains correct and prevents regression bugs.