Gibbs / Metropolis update of the latent normal matrix Z underlying a
binary relational matrix Y. Off-diagonal entries are drawn from the
dyadic full conditional \(Z_{ij} \mid Z_{ji} \sim N(EZ_{ij} + \rho\,
(Z_{ji}-EZ_{ji}),\; 1-\rho^2)\) truncated to the half-line implied by
\(Y_{ij}\) (positive when \(Y_{ij}=1\), negative when \(Y_{ij}=0\),
unrestricted when \(Y_{ij}\) is missing). A correlated dyad-level
Metropolis proposal is then attempted to improve mixing, and the diagonal is
refreshed from its unconstrained conditional.