RADIX_MULMID_FFT_CUTOFF_RATIO

The tuning ratio of radix_mulmid. With fft_small it is FLINT_RADIX_MULMID_FFT_SMALL_CUTOFF_RATIO, which every flint-mparam.h must define: 50 on Zen 3 and Zen 4, 60 on Ice Lake and later Intel cores and in the generic files, 80 on Skylake and Broadwell, 85 on arm64 (measured on Apple M4). The measured crossover is the same for balanced full products, Newton middle products and short windows of a long operand; a balanced full product of n limbs switches to the FFT at n = 2 \cdot ratio, 100 to 170 limbs. Without fft_small the ratio is 175 (Kronecker substitution pads each limb to two words, so the crossover for a balanced full product is at about 350 limbs).