The native handle is a process-local cache of the fitted engine model,
created by lmm() and glmm(). While it is alive, computations that need
the engine model (contrasts, summary() tests, predictions with intervals,
ranef(condVar = TRUE), profiles, bootstraps, model comparison,
verify_convergence()) reuse it instead of refitting. A FALSE result
does not mean the fit is unusable: the handle does not survive
saveRDS() / readRDS() or a new R process, and those computations then
refit the model from the stored model frame, with identical results;
extractors read from the durable artifact and flat R-side payload, and
revive() recreates the lazy cache after serialization. Set
options(mixeff.keep_handle = FALSE) before fitting to not keep handles
(each one holds a copy of the model data for as long as the fit object
lives).