Lips on rounded, shelled rims: the band along the rim's edge is raised from the body's own faces (kernel::raise_band: the rim face split along the band's exact offset edge, the band's walls swept up from the walls below, a top), since a join can't merge walls that continue each other; grooves cut with a band reaching into the free side so no wall coincides; tests against the band's area times its height

Fixes #29

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RjGw4gA5ZvKrU8MQP6sBgT

crates/kernel/src/offset.rs

@@ -423,7 +423,7 @@ fn span_step(ns: &[Vec3], s: &[f64]) -> Option<Vec3> {
 }
 
 /// A line or circular arc through an edge (by samples): `None` for other curves.
-fn arc_mid(e: &mt::Edge, tol: f64) -> Option<Option<Vec3>> {
+pub(crate) fn arc_mid(e: &mt::Edge, tol: f64) -> Option<Option<Vec3>> {
     use mt::{BoundedCurve, ParametricCurve};
     let c = e.curve();
     let (t0, t1) = c.range_tuple();