Add sliding window maximum using monotonic deque

[P09s]

Implements the classic sliding window maximum problem (LeetCode 239) using a monotonic deque for O(n) time.

Features:

• Full Google-style docstring with examples and complexity analysis
• Type hints
• Doctests for normal + edge cases
• Clean, educational code following repo standards

Ready for review!

[P09s]

Hi maintainers! @poyea @cclauss @mindaugl

• Updated the implementation with modern type hints (list[int], deque[int]) to satisfy Ruff rules (UP006/UP035)
• Fixed end-of-file newline
• All CI checks are now green (ruff, pre-commit.ci, build, docs)
• Tests pass, label removed automatically

Ready for review! Would appreciate any feedback
Thanks!