arb interval powers with integer exponents can return nan #426

Description

Raising an arb interval that crosses zero to an exact integer exponent can return nan, even when the power has a finite real enclosure. This affects the released 0.9.0 wheel and the current arb.__pow__ implementation.

Reproducer

from flint import arb

x = arb(0, 1)
print(x)       # [+/- 1.01]
print(x ** 1)  # nan
print(x ** 2)  # nan

Observed with python-flint 0.9.0 and FLINT 3.6.0.

Expected behavior

Both results should be finite enclosures. In particular, squaring an interval that contains [-1, 1] should contain both 0 and 1.

Cause

arb.__pow__ currently converts every supported exponent to an arb and calls arb_pow. That discards the fact that Python int and fmpz operands are exact integers. FLINT exposes arb_pow_fmpz specifically for integer exponents, and using it also avoids the zero-crossing failure in generic real exponentiation.

The FLINT power API documents arb_pow_fmpz as integer binary exponentiation: https://flintlib.org/doc/arb.html#c.arb_pow_fmpz

I have a focused patch with regression coverage for Python int and fmpz exponents.