Mirror image of rZ_ord_sym_fc (variance 1 on the upper
triangle, mirrored to the lower) parameterised by an explicit
alpha vector instead of data-induced cutpoints.
rZ_ord_sym_explicit_fc(Z, EZ, Y_int, alpha)updated symmetric Z (diagonal sampled from unconstrained prior)
current symmetric latent matrix
expected value of Z
integer ordinal Y (symmetric)
length-(K-1) cutpoints with alpha[1] = 0