One Carter-Kohn sweep over the per-period observations of the form
vec(E_t) = H_t g_t + eps_t, with state transition
g_t = rho * g_{t-1} + eta_t.
ffbs_vecG(E_cube, U_cube, V_cube, s2, rho_G, sigma_G2)list with G_cube (RA x RB x T) and vecG_path
(p x T draws of the vectorised state).
nA x nB x T residual cube (Z minus base - a - b - UV')
nA x RA x T latent row factor cube
nB x RB x T latent column factor cube
scalar observation variance
AR(1) coefficient in (-1, 1) (use rho = 1 for
the RW1 limit; the forward variance is clamped via a tiny floor).
state innovation variance