Unpack NF consecutive fields of one u32 word starting at field first (a runtime index),
cast but unscaled — the caller multiplies by whatever scale its lines carry. The sub-word
counterpart of [unpack_cast_u32], for reads whose served line is narrower than a word:
NF may be any divisor of the packing factor, and first selects which slice of the word
this line is. e2m1 is refused — its native pairs cannot be split at a field boundary.