Preserve Duration state in pickle and shallow copies

[GreedyC]

Fixes #1023.

Preserve Duration's instance state alongside the existing timedelta reduction. Pickle round-trips and shallow copies then retain the year/month components rather than turning them into approximate weeks/days. Interval's existing reduction is unchanged.

Pull Request Check List

• Added tests for changed code.
• Updated documentation for changed code. No new API: restores the existing value's calendar components.

Validation:

• 19 regression cases fail before the fix, covering every pickle protocol and positive/negative/mixed-sign calendar components.
• Pure-Python suites on Python 3.10 and 3.12: 1,868 passed, 3 skipped each.
• Pinned Ruff 0.14.11 lint/format checks pass; mypy 1.13.0 passes for all 228 source files.
• The Rust extension variant was not run locally. The change is confined to Python Duration reduction.

AI assistance: Codex.