[rupayon123]

Fixes #24. Duration's comparison/arithmetic methods raise NotImplementedError for unsupported operands, preventing Python from using the other operand's reflected method. Return NotImplemented instead. Unrelated equality now returns false, custom reflected operations can participate, and unsupported arithmetic raises the standard TypeError. Existing datetime arithmetic is unchanged.

All six regression cases fail before the fix. Afterward, all 382 tests pass, with 100% reported source/branch coverage in a separate coverage run. Scoped Black/Flake8, strict source mypy, and git diff --check pass on Python 3.12.13. The __sub__ return annotation uses Any rather than incorrectly claiming None, retaining compatibility with the project's Python 3.7 minimum instead of importing newer types.NotImplementedType at runtime.

Prepared with AI assistance; Python operator fallback and existing datetime cases were verified.