Support native timedelta operands in duration arithmetic

[muneikh]

Dividing a Duration by datetime.timedelta, or using //, % or divmod, raises AttributeError because the native operand has no _to_microseconds() method. Convert native timedelta days, seconds and microseconds with integer arithmetic while retaining the existing Duration path. This also lets a zero native divisor raise the expected ZeroDivisionError.

Fixes #382. Calendar month/year arithmetic described in #799 is outside this fix.

Sixteen new regression cases fail before the fix. The full suite passes (1,886 tests, 3 skips); the duration/interval subset passes (105 tests), and pinned Ruff lint/format checks and focused Mypy pass.

• Added tests for changed code.
• Updated documentation for changed code (the existing timedelta arithmetic contract is retained).