Single internal entry point for all Monte Carlo fallback paths.
If x is already an empirical_dist, returns it unchanged.
If x has a .cache environment (e.g. edist objects),
caches the realization so that multiple method calls (e.g. cdf +
density) share the same samples. Sample-size-aware: if the cached
realization has fewer than n samples, re-realizes.
ensure_realized(x, n = 10000L)An empirical_dist (or realized_dist).