Fix boss-bottom blends and merged arc-corner surfaces (#1294)

A fillet or chamfer on a cylindrical boss's bottom arc now builds with a closed display mesh: the seam-extension containment ray counts a shared curved-boundary endpoint once. Merging the split faces of a three-edge arc corner rebuilds the face on the original input face's full surface instead of a piece's restricted patch, and runs under the kernel's panic guard. Fixes #1262 and #1263.

Co-authored-by: Karl-VonRichter <20079846+Karl-VonRichter@users.noreply.github.com>

crates/kernel/src/curveblend.rs

@@ -455,7 +455,7 @@ pub(crate) fn project(f: &mt::Face, p: Vec3, hint: Option<(f64, f64)>) -> Option
 }
 
 /// Whether `uv` lies within the parameters of the face's surface (unbounded directions pass).
-fn in_range(f: &mt::Face, uv: (f64, f64)) -> bool {
+pub(crate) fn in_range(f: &mt::Face, uv: (f64, f64)) -> bool {
     use mt::ParametricSurface;
     use std::ops::Bound;
     let inside = |range: (Bound<f64>, Bound<f64>), x: f64| {