Fix Argument typing, modernize to 3.10+, reserve internal attr names

[mosquito]

Summary

Three related typing/quality improvements:

1. Argument() no longer infers as Any. The fallback overload now
   returns a bare TypeVar, so annotated attributes adopt their declared
   type — tags: list[str] = Argument(nargs="+") infers list[str] —
   silencing basedpyright's reportAny on the common untyped form.

2. Modernized the library to PEP 604/585 (3.10+): Optional[X] → X | None,
   Union → |, List/Dict/Tuple/Type → builtins, typing.Callable
   /Iterable/Mapping → collections.abc. Enabled ruff's UP rules.
   Tests are exempt (they intentionally exercise legacy typing forms to
   verify backward compatibility).

3. Reserved internal attribute names. current_subparsers,
   current_subparser, and __parent__ now raise ArgumentDefinitionError
   when used as user argument/group/subparser names, instead of silently
   clobbering parser state. This let the load-bearing type-comment on
   AbstractParser.current_subparsers become a normal annotation.

Tests & docs

• New tests/test_reserved_names.py (10 tests).
• Documented reserved names in docs/subparsers.md + a pitfalls.md cross-reference.

Verification

• ruff check clean, ruff format clean.
• mypy clean (only the pre-existing tomllib stdlib-stub baseline).
• 655 tests pass; doc code blocks pass; Sphinx builds without warnings.