fix: add the stdlib fold argument to Time and DateTime

[tuyentran4992]

Summary

Fixes #619: Time and DateTime do not accept the stdlib fold argument.

• item(time(12, 34, 56, fold=1)).fold returns 0 - the fold is silently dropped (same for datetime);
• item(time(12, 34, 56)).replace(fold=1) raises TypeError on CPython 3.12 (measured);
• on pure-Python implementations (PyPy, per this issue's traceback) replace() calls type(self)(..., fold=fold), which these constructors reject.

Changes

• Time.__new__/__init__ and DateTime.__new__/__init__ accept fold: int = 0 - appended after raw, so existing positional call sites are unaffected, and callers pass it by keyword (as the stdlib does); it is forwarded to the stdlib constructor;
• Time._new/DateTime._new carry fold over in replace() results;
• item() forwards value.fold when converting datetime/time values.

Tests

Two tests added in tests/test_items.py (test_times_behave_like_times_fold, test_datetimes_behave_like_datetimes_fold) cover construction, item() and replace() round-trips.

Verified on a clean checkout with this patch applied via git apply:

• tests/test_items.py -> 94 passed
• full suite -> 1060 passed in 3.23s (Python 3.12, pytest only, unprivileged sandbox)

Agent Drafting Metadata

• Agent: @tuyentran4992's assistant agent, using FixFork - a dev-stage debugging tool (https://github.com/tuyentran4992/fixfork) - to reproduce the issue and draft fix candidates.
• Model: nvidia/nemotron-3-super-120b-a12b (Nebius Token Factory) inside the tool's runs.
• Notes: the tool's live runs on this issue are public (reports, raw model replies, event logs): https://github.com/tuyentran4992/fixfork/tree/master/examples/tomlkit-619 - they did not produce the final patch end-to-end; the patch above was completed and verified by hand against tomlkit's full suite before submission.