Extend the region-type variety beyond box/ball with Ellipsoid (axis-aligned,
per-axis semi-axes -- the anisotropic / Gaussian-covariance member). Exact
contains, exact log_volume, and an exact Newton-iterated point-to-surface
distance (solves x_i = a_i^2 q_i/(a_i^2+lambda) for the boundary lambda);
region-to-region predicates use the bounding box as a documented approximation
(exact ellipsoid-ellipsoid containment/intersection-volume has no closed form).
It implements Region, so it drops into RegionIndex like the others.

Also a containment-recall regression test: on a realistic hierarchy of big
general boxes (scattered, far centers) with small nested boxes, the lifted
containing() recovers enclosing regions at >95% recall vs exhaustive -- the
quantified, in-CI version of the far-centered-enclosure guarantee.

Additive (new type + test); no API break.