Preserve dict LIFO order in Box.popitem

[machines-6657]

Fix Box.popitem() to follow dictionary LIFO ordering

Box is documented as a near drop-in replacement for dict, but Box.popitem() removed the first inserted key while Python's dict.popitem() removes the most recently inserted key (LIFO, guaranteed since Python 3.7). The existing test used a one-item Box and could not detect the ordering mismatch.

This patch changes Box.popitem() to select the last key through Box's reverse iterator and then removes it through Box.pop() so the existing key bookkeeping remains in effect. The regression test checks two ordered removals and preserves the empty-Box exception assertion.

Validation: the new regression assertion failed before the implementation change and passes after it. The full test suite, repository pre-commit hooks, mypy, Windows Cythonized test hook, wheel build, and twine check pass locally. The test suite was run with PYTHONUTF8=1 because this Windows host otherwise defaults to GBK and cannot decode an existing TOML fixture.

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