mesh: an enclosure's corner bosses are ringed where its walls join them (#402)

A boss the body's walls run into had its circle crossed by the walls' planes, so the projection
layout refused it and the plan fell back to swept cells, fanning round the boss and its hole.
Such a boss is now framed by the square round its whole circle, subdivided by the planes that
cross it; every square inside (the hole's, the core) is divided alike, so the ring's blocks meet
the walls node to node. On the walls' side the frame stops at their inner plane and its corner
projects onto the middle of the inner fillet, which the ring takes in (a boss just touching the
fillet included: no sliver). Cylinders a fillet's history names but that are a ring's circle are
the ring's faces. Rings are built per patch of grid cells, the one-cell O-grid being the
simplest patch.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

skills/fieldcraft-meshing/SKILL.md

@@ -35,8 +35,9 @@ not guessed. Read it before you plan anything:
    body did not fit (a plane at an angle, a curved face that is not a ring or a blend, two
    circles in one frame), and then the frame cuts and swept cells are used instead (`pending`
    lists the rings left boxed). Check `all_hex`, `conforming`, `min_scaled_jacobian`.
-   Known gap: a shelled box whose fillet radius exceeds the wall (the enclosure-base pattern)
-   can still leave a twisted element at the outer fillet's bottom corner.
+   A boss the walls run into (an enclosure's corner screw boss) is ringed over the square round
+   its whole circle, the ring split where the walls join it; the inner fillet where the walls
+   meet is taken in by the ring's corner, even where the boss just touches it.
 4. Then verify as in §4. When the plan does not fit (unmapped primary features, a failed cell),
    fall back to the manual process below, keeping the plan's cuts that work.