Fix scalar color type hints for FadeIn and FadeOut

[Sev7een]

Summary

• allow FadeIn.initial_color and FadeOut.final_color to be typed as scalar floats for mask clips
• keep tuple and list color support explicit for regular clips
• add regression coverage for scalar mask fade colors and their public type hints

Root cause

Both effects already support scalar colors at runtime because they convert the provided color with numpy.array. Their docstrings also document scalar mask colors, but the dataclass annotations only allowed list. As a result, valid calls such as FadeOut(duration, final_color=0.0) were rejected by static type checkers.

Closes #2472

Validation

• python -m pytest tests/test_fx.py -q -k "fadein or fadeout"
• python -m pytest
• python -m pytest --doctest-glob "moviepy/**/**.py"
• black --check --diff --color .
• isort --check-only --diff moviepy docs/conf.py examples tests
• flake8 -v --show-source --ignore=E501 moviepy docs/conf.py examples tests

AI assistance disclosure

This contribution was prepared with Codex assistance. The final diff and validation results were reviewed locally before submission.