Preserve floating-point results of negative integer powers

[fhgffy]

Summary

Negative powers of integer items currently disagree with their serialized value: item(2) ** -1 compares equal to 0, but renders as 0.5. Fractional exponents also raise instead of allowing Python's reflected float operation.

Keep floating-point power results as Float items and pass NotImplemented back to Python for reflected dispatch. Positive and modular integer powers retain their existing result type.

Tests: 12 new cases pass (7 fail on the original implementation); the complete suite passes with 1109 tests on Python 3.12. Ruff and formatting checks pass, and both wheel and sdist build. Mypy reports the same five pre-existing errors on the original and patched source.

The downstream Poetry integration fails one extraction assertion on Python 3.12.15. A controlled replay with the official tarfile modules reproduces the difference without importing tomlkit: 3.12.15 normalizes the path and writes safely inside the destination, while the unchanged Poetry test expects OutsideDestinationError. The test expectation is addressed in python-poetry/poetry#11133.

Agent Drafting Metadata

• Agent: Codex
• Model: GPT-6
• Notes: Assisted with investigation, the patch and regression tests.