Samples the posterior of a model assembled by 'frmtmb', by handing its 'RTMB' objective to 'tmbstan' and so to Stan's NUTS sampler. Applies the 'brms' default priors, samples the qualifying random-effect blocks in their non-centered form, and returns draws under 'frmtmb' parameter names. Provides the posterior method surface those draws support: expectations and predictions, pointwise log-likelihood and approximate leave-one-out cross-validation, hypothesis tests, conditional effects, posterior predictive checks, and conversion to the 'posterior' and 'coda' draws formats.