feat: fillet radius tapers along an edge to the room there (#749)

Adds `fit: taper` to Fillet: where a constant radius doesn't fit along a straight edge, it follows the room there as a smooth spline, down to zero where a face closes to a point. Taper to Fit is in the Fillet dialog, and the feature's warning says how far it tapered.

crates/kernel/src/lib.rs

@@ -79,7 +79,7 @@ mod untouched_faces_tests;
 
 pub use blend::{ChamferSide, chamfer, chamfer_sides, fillet, fillet_each, fillet_max, fillet_max_each};
 pub use blend_corner::fillet_radii;
-pub use blend_variable::{fillet_chord, fillet_variable, fillet_variable_points};
+pub use blend_variable::{RadiusSpline, Taper, fillet_chord, fillet_tapered, fillet_variable, fillet_variable_points, fillet_variable_spline, taper};
 pub use body::{Body, CapSignature, DISPLAY_ANGLE, EdgeInfo, FaceInfo, FacePaint, FaceSource, Paint};
 pub use boolean_diagnostics::{BooleanDiagnostics, BooleanFallback, BooleanStrategy, capture_boolean_diagnostics};
 pub use build::{PathSeg, box_solid, canonical_revolve_axis, cylinder, extrude, extrude_tapered, loft, revolve, sphere, sweep, torus};