A pre-simulated panel; when omitted, one is generated from dgp_args (which accepts either W or grid=(nrow, ncol), plus the :func:~scspill.simulate.dgp.scspill_sim_dgp arguments; alpha defaults to the paper’s planted weights).
Seeds the replication. The DGP (when generated here) and the samplers draw from two independent child streams of this seed, so the simulated data and the MCMC noise are never correlated.