Initial release: a portable Stan-backend layer that a Stan-based R package can fit its models through, using either rstan (default) or, optionally, cmdstanr.
stan_options() collects and validates sampler options for the chosen
backend, forwarding them verbatim and guarding against mixing one backend's
argument vocabulary into the other.fit_model() dispatches a fit to the backend recorded on the options and
resolves the calling package's compiled model automatically.backend_draws_array(), backend_extract(),
backend_generate_quantities(), and backend_has_draws().use_flexstanr() wires flexstanr into a host package's DESCRIPTION.requireNamespace()
guards; rstan is the default and only hard Stan dependency.