Missingness posterior stress

Opt-in Gaussian MAR nested-MC imputation (impute_posterior) pooled into run_lmtp_grid(...; imputation=draws) under Rubin's rule. Cheap :drop / :ipcw remain defaults; Turing / RxInfer backends are deferred.

Quarto: docs/stress/missingness_posterior_stress.qmd

cd docs/stress && quarto render missingness_posterior_stress.qmd

Structural claims come from CausalDynamics MissingnessSpec / certify_missingness (see identify(...; missingness=)).