The vec_dot pairs DT3 with Q8_0 (4 q8_0 blocks per DT3 block) and keeps
one integer accumulator per plane: sumf += dy * (d1*sumi1 + d2*sumi2).
Q8_0 instead of Q8_K on purpose: the planes are symmetric ternary so the
q8_K bsums are dead weight, and 32-element blocks accept any row size
that is a multiple of 128.
Trit decoding reuses unpack_plane_dt3, which reads only 4 trits per qh
byte; the 5th base-3 digit of those bytes is packer padding that always
decodes to -1 and must never be read.
Millaguie
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