Conform Int128 to FixedWidthRationalElement, gated on compiler(>=6.0)
and the platform availability where the type ships.

Override squareRoot/formSquareRoot for FixedWidthRationalType instead
of inheriting the RationalType default, which shifts numerator and
denominator by 2*max(bitWidth) and overflows immediately on a fixed
width integer. The override uses sqrt(n/d) == sqrt(n*d)/d, falling
back to Double for small values.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>