Proposes rho* = tanh(atanh(rho) + tau * N(0,1)); under a uniform
rho ~ U(-1, 1) prior the proposal is symmetric in the Fisher-z
scale and the Jacobian is log(1 - rho^2). The MH ratio
compares the AR(1) prior density of vec(G)_path at the
current vs proposed rho.
sample_rho_G_mh(rho_G, vecG_path, sigma_G2, tau = 0.3)list with rho (updated), accept (logical)
current AR(1) coefficient
p x N matrix of FFBS-sampled states
state innovation variance
RW proposal SD on the Fisher-z scale