fix(kernel): name the zero-thickness rim when a shell can't open beside a round (#316)

Shelling a body through a face that a convex round meets tangentially, with
the round at least as large as the thickness, puts the cavity wall through
the rim where the round meets the open face. Both shell strategies then fail
and the first one's generic error ("a corner whose faces don't meet the same
way after the move") was reported. The shell now says why and what to change.
Successful shells are unchanged.

Found by fuzz seed 33, which reaches this case now that a disc's two edges can
be rounded to half its thickness.


Claude-Session: https://claude.ai/code/session_01TWc1smHYwTSVrVc8SNvS44

Co-authored-by: Claude Opus 5.5 <[email protected]>

crates/kernel/src/offset.rs

@@ -42,7 +42,7 @@ fn tube_centre(o: Vec3, a: Vec3, big: f64, h: f64, p: Vec3) -> Option<Vec3> {
 
 impl Surf {
     /// Outward unit normal of the surface at `p` (on or near it).
-    fn normal(&self, p: Vec3) -> Option<Vec3> {
+    pub(crate) fn normal(&self, p: Vec3) -> Option<Vec3> {
         match *self {
             Surf::Plane { n, .. } => Some(n),
             Surf::Cylinder { o, a, convex, .. } => {