Add Newton (reciprocal based) division for large quotients#80
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Division via an approximate reciprocal of the divisor (Brent-Zimmermann's precision-doubling Newton iteration) followed by Moller-style quotient-block development with a bounded correction loop. It is dispatched alongside schoolbook and Burnikel-Ziegler when both the divisor and the quotient exceed ~6000 words, where it is up to ~2x faster than Burnikel-Ziegler (see the new ubig_div_asymmetric benchmark; the existing symmetric ubig_div stays on the schoolbook path). Also fixes a latent memory-sizing bug in gcd_ext post-processing (it sized the scratch for the wrong divisor length, exposed by Newton's larger requirement), and adds a defensive fallback so callers that under-size scratch transparently use Burnikel-Ziegler. Co-Authored-By: Claude <noreply@anthropic.com>
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Division via an approximate reciprocal of the divisor (Brent-Zimmermann's precision-doubling Newton iteration) followed by Moller-style quotient-block development with a bounded correction loop. It is dispatched alongside schoolbook and Burnikel-Ziegler when both the divisor and the quotient exceed ~6000 words, where it is up to ~2x faster than Burnikel-Ziegler (see the new ubig_div_asymmetric benchmark; the existing symmetric ubig_div stays on the schoolbook path).
Also fixes a latent memory-sizing bug in gcd_ext post-processing (it sized the scratch for the wrong divisor length, exposed by Newton's larger requirement), and adds a defensive fallback so callers that under-size scratch transparently use Burnikel-Ziegler.