The opening's measured sightline crosses two layers of the same wall, and
only the inner layer was kept, so the outer one still closed it. The keep
is now a region around the opening, mirrored to defense like the reviewed
ones, and a test pins the perch-to-crate sightline.

Review fixes to the nudge: an open-floor exit beyond reach no longer stops
the step across the nearest edge, and only edges within reach are sorted.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>