_padic_radix_poly_mulmid_KS, _padic_radix_poly_mulmid_mod

Middle product by Kronecker substitution in radix $B$: the coefficients, scaled to the base frames of the two polynomials and truncated to the largest output window of $S$ limbs, are placed in slots of $2S + O(1)$ limbs of a single radix integer, and one radix_mulmid (which uses the small-prime FFT directly in radix $B$ for large operands, without conversions to binary) gives every output in its own slot. The mod version instead multiplies the residues modulo $B^S$, avoiding the padding of the slots: with _nmod_poly_mulmid when $S = 1$ and with _mpn_mod_poly_mulmid (classical, Karatsuba or the small-prime FFT, as tuned for mpn_mod) when $B^S$ has at most MPN_MOD_MAX_LIMBS binary limbs, converting to binary and back; otherwise it returns GR_UNABLE. The output precisions come from min-plus convolutions of the exponent sequences (below), and coefficients whose terms are all exact are summed exactly (in windows covering the extents of the terms) when padic_radix_dot_strided_delayed would keep them exact. The results are those of _padic_radix_poly_mulmid_packed, except with the context flag PADIC_RADIX_FAST_BOUNDS, which allows smaller (but never larger) precisions. Both return GR_UNABLE in the same cases as the packed version. Outputs whose window in the base frames exceeds four times (plus 8 limbs) the largest window of an output in its own frame (all of whose terms have valuations far above the base frames, e.g. next to a few coefficients of huge valuation) are computed separately by padic_radix_dot_strided_delayed when there are few of them (costing at most about the transform and a sixteenth of the classical algorithm); otherwise, that is when the valuations vary beyond the precision along the outputs (alternating, trending or random valuations spread over more digits than the precision), both return GR_UNABLE. High-valuation coefficients that are beyond the precision of every output they contribute to cost nothing.