refactor: centralize and test score arithmetic

[yanglluo]

What changed

• Extracted score arithmetic into a pure calculate_total_score helper.
• Kept category caps, bonus points, deductions, and the overall maximum deterministic.
• Added regression tests for category caps, deductions, and the total-cap branch.

Validation

python -m unittest -v test_scoring.py
Ran 3 tests — OK (including the total-cap branch)

python -m py_compile scoring.py test_scoring.py score.py
PASS

This is intentionally a small refactor: it improves testability without changing the scoring contract.