One full sweep over actors: each actor's path is sampled from its conditional AR(1) posterior via .actor_ffbs_path(), and the joint sample is then projected to satisfy sum_i theta_i(t) = 0 per period using the variance-weighted Lagrangian.
.sweep_per_actor_exact(H_mat, h_mat, rho_actor, sigma_actor2)n_actors x T matrix of centered draws.
n_actors x T precision sufficient statistics
n_actors x T cross-product sufficient statistics
AR(1) coefficient
AR(1) innovation variance