The temporal analogue of dispatch_gibbs_spatial(): maps a validated
temporal_multiscale() spec onto the multiscale temporal Polya-Gamma
sampler (cpp_pg_binomial_gibbs_temporal), which composes an additive
RW1 trend + cyclic-RW1 seasonal + AR1/IID short-term decomposition. Binomial
only. The C++ kernel implements an RW1 trend (rw2 is rejected here rather than
silently downgraded).