Fix proofs (py3.11) crash harness CLI parsing for empty positional args

[Copilot]

proofs (py3.11) was failing before executing crash proofs because tests/crashtest.py rejected the default no-argument invocation (python tests/crashtest.py) during argparse validation.
This change makes argument handling Python 3.11-compatible while preserving strict validation for explicitly provided crash points.

• Root cause

  • argparse positional POINT was defined with nargs="*" and choices=POINTS.
  • On Python 3.11, empty positional input was treated as an invalid choice path for this configuration.

• Change

  • Removed choices=POINTS from the variadic positional argument definition.
  • Added explicit post-parse validation for user-provided points and surfaced the same invalid-choice style error message.

• Behavioral impact

  • python tests/crashtest.py now correctly runs all crash points by default.
  • Invalid point names still fail fast with a clear CLI error.

parser.add_argument("points", nargs="*", metavar="POINT", ...)

invalid = [point for point in args.points if point not in POINTS]
if invalid:
    parser.error(
        "argument POINT: invalid choice: "
        f"{invalid[0]!r} (choose from {', '.join(repr(point) for point in POINTS)})"
    )

[Antrikshgwal]

LGTM