Keep fine meshes whole at collapsed taper boundaries (#906)

The exact surface search could return an arbitrary parameter at a collapsed NURBS boundary, which corrupted the trimmed triangulation domain and lost volume on fine meshes of a fillet tapering to zero. Search results in the boundary projection are now validated, preferring a bounded parameter at a pole and snapping to a finite endpoint only when its transverse tangent has collapsed and the residual improves. Adds four regressions for decreasing chord tolerances, pole parameters, unbounded planar support and provisional preview.

Fixes #796.

crates/kernel/src/blend_variable.rs

@@ -819,3 +819,7 @@ mod tests {
         assert!(rel(measure(&f).unwrap().volume, measure(&k).unwrap().volume) < 1e-9);
     }
 }
+
+#[cfg(test)]
+#[path = "taper_mesh_tests.rs"]
+mod taper_mesh_tests;