* Nystrom approximation as an explicit `approx = "nystrom"` mode, with
  conditional approximate inference under `vcov = TRUE`.
* AME-variance computation rewritten via the row-sum identity for an
  O(n^2) per-predictor cost in the default exact path.
* Bug fixes: `tol` propagated to `lambdasearch()`; `drop = FALSE` on
  single-eigenvector `eigtrunc` subsets.