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 <[email protected]>
Claude-Session: https://claude.ai/code/session_01AS5Tn3xieewUSg3uxrMVdL

crates/engine/src/cmd/sketch_project_tests.rs

@@ -296,7 +296,8 @@ fn isoparametric_curves_of_a_cylinder() {
     let w = &sk.wires[sk.wire_index(r["wires"][0].as_str().unwrap()).unwrap()];
     let zs: Vec<f64> = w.pts.iter().map(|p| p.z).collect();
     assert!(zs.iter().cloned().fold(f64::MIN, f64::max) > 19.9 && zs.iter().cloned().fold(f64::MAX, f64::min) < 0.1, "{zs:?}");
-    assert!(w.pts.iter().all(|p| (p.x - 6.0).abs() < 1e-3 && p.y.abs() < 1e-3));
+    // Within the mesh's sag: the cut runs across facets unless a seam happens to lie there.
+    assert!(w.pts.iter().all(|p| (p.x - 6.0).abs() < 1e-2 && p.y.abs() < 1e-2), "{:?}", w.pts);
 }
 
 #[test]