[egorsmkv]

pickle.dumps() raises TypeError for fasttime.DateTime and fasttime.Duration. Add PyO3 __reduce__ methods that restore values through existing public constructors: integer i128 nanoseconds for Duration, and whole Unix seconds plus a nonnegative nanosecond component for DateTime. This preserves nanosecond precision and negative timestamps without float conversion or mutable restore state.

Closes #34.

Changes:

• Add regression tests in a separate first commit; reproduce both failures before implementing the fix.
• Cover protocols 0–5, default and -1; i128 duration limits; i32 year limits; year zero and leap days; -1 ns; equality, hashes, dictionary/set membership; clean-process reconstruction; and shared-instance thread use.
• Update type stubs and Python documentation.
• Extend Linux CI to Python 3.15, 3.14t and 3.15t, checking that importing the extension keeps the GIL disabled. Existing Windows wheel tests also exercise the regressions.

Validation:

• All 200 Python tests pass locally on CPython 3.10.22, 3.12.14, 3.14.8t and 3.15.0rc3t. Both free-threaded interpreters retain the disabled GIL.
• Rust tests pass with default features, no default features and the Python feature (20 tests in each configuration; Python-feature Rust tests use CPython 3.10).
• Rust formatting, Clippy including Python bindings, Ruff lint/format, Pyright and all five Python examples pass.
• Test workflow: successful, including all eight Linux Python configurations (3.10–3.15, 3.14t, 3.15t), Rust and quality checks.
• Python distribution workflow: all eight Windows wheel build/tag/test jobs successful, including 3.14t and 3.15t. Release-only jobs are skipped for pull requests.