Robustness runs and the kernel fixes they found

- solvecraft-cli fuzz / fuzz-one: seeded random modelling sessions (boxes, extruded profiles with arcs, revolves; joins and cuts of boxes, cylinders and spheres, fillets, chamfers, shells, holes, patterns) on a grid that makes coincident faces and tangencies common, every step checked (command ok, every body a sound solid via the new Body::validity, volume moving the right way), plus the boolean volume identity on random primitives; one process per seed with a time limit; failures minimised to scripts
- shell/offset: corners where a curved face meets another at an angle are refined onto the moved surfaces (Newton); edges that are neither lines nor arcs are rebuilt through their moved points; one shell per piece; faces that would cross say so (not supported yet) instead of leaving a cavity inside out; flat revolved faces count as planes; the unhealed body is tried when the healed one fails
- cylinders start their seams at an odd angle (grid-aligned seams broke booleans along them)
- partial revolves of profiles touching the axis build no zero-area face on the axis
- booleans on bodies in several pieces retry a piece at a time
- regression tests for each, and a fixed set of seeds in cargo test

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AS5Tn3xieewUSg3uxrMVdL

crates/kernel/src/meshbody.rs

@@ -147,7 +147,7 @@ fn unify_winding(m: &mut TriMesh) {
 }
 
 /// Every triangle side is shared by exactly two triangles in opposite directions.
-fn is_closed(m: &TriMesh) -> bool {
+pub(crate) fn is_closed(m: &TriMesh) -> bool {
     let mut count: HashMap<(u32, u32), i32> = HashMap::new();
     for t in &m.triangles {
         for (a, b) in [(t[0], t[1]), (t[1], t[2]), (t[2], t[0])] {