Fix arb powers with integer exponents

[morluto]

Fixes #426.

arb.__pow__ currently converts Python and FLINT integer exponents to arb before dispatching to generic arb_pow. For an interval crossing zero, that path can return nan even for exponents such as 1 and 2.

This change preserves the exponent type and calls FLINT's integer-specific arb_pow_fmpz for Python int and fmpz operands. Other supported exponent types continue to use arb_pow as before. Regression coverage checks that zero-crossing interval powers remain finite and enclose representative values.

Validation:

• spin test -- --pyargs flint.test.test_arb -q (47 passed)
• spin test -- -q -k "not docstrings" (456 passed)
• cython-lint src/

The full doctest collection was also attempted under a local CPython 3.14 free-threaded build; it has pre-existing doctest namespace failures unrelated to this change, so the passing general and focused suites above are the relevant evidence.