Add the dual-plane ternary DT3 type to the type registry along with its
reference row functions. Each of the two planes is packed exactly like
tq1_0 with all constants halved (block of 128 elements): qs 48 -> 24
bytes over two passes of 16 and 8 bytes, qh 4 -> 2 bytes.
The trit decoding lives in a single exported helper (unpack_plane_dt3)
so that dequantization and the upcoming CPU vec_dot share it.
The reference quantizer is a greedy two-pass (plane 1 by absolute max,
plane 2 on the residual) plus two rounds of alternating least-squares
refits. It is intentionally NOT the PTQTP solver used to produce the
published DT3 models.
Millaguie
160ea6c428