fix(lists): support general iterables and update typing in natural_list

[Pxnzerr]

Summary

natural_list() documented its argument as items (Iterable): An iterable of items, but:

1. The type annotation was restricted to items: list[Any].
2. The implementation relied on sequence indexing (items[0], items[:-1], items[-1]) and len(items), raising TypeError when passed generators, sets, dict_keys, iterators, or custom non-subscriptable iterables.

Changes

• Updated natural_list type annotation to items: Iterable[Any] (importing Iterable in TYPE_CHECKING).
• Normalized items using list comprehension (item_list = [str(item) for item in items]), safely handling any finite iterable in a single pass.
• Added comprehensive unit test coverage in tests/test_lists.py for tuples, sets, generators, dict_keys, and range.

Validation

• pytest tests/test_lists.py: 14 passed
• pytest: 706 passed
• mypy src tests/test_lists.py: clean
• ruff check .: clean