Integrate the MXFP4 Metal decode/prefill optimizations (rounds 7-9) as one
commit. Keep both the MXFP4 half_lut/tail_cull down-pipeline selection and
main's mpp_mask override for IQ2_XXS gate/up and Q2_K/IQ2_XXS down; they
target disjoint quant types and coexist.