Fillet/chamfer: point the drag arrow into the corner on inside edges (#1394)

* Fillet/chamfer: point the drag arrow into the corner on inside edges (#1384)

The radius/distance arrow always pointed against the faces' outward
bisector, which is into the body on an outside edge but away from the
corner on an inside one, where the blend adds material. The arrow and
the spline radius handles now take each edge's convexity into account,
so dragging out along the arrow grows the value on both kinds of edge,
and each edge of a mixed chain follows its own.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* Tests: no unreachable! in the blend arrow helper

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

crates/ui-egui/src/lib.rs

@@ -89,6 +89,8 @@ mod file_load;
 mod file_picker;
 pub use file_picker::FilePickerFuture;
 #[cfg(test)]
+mod blend_arrow_tests;
+#[cfg(test)]
 mod file_load_ui_tests;
 #[cfg(test)]
 mod fillet_spline_handle_tests;