fix: a size worked out from a bare diameter keeps its unit when the design's units change (#716) (#779)

A size typed as a bare diameter now keeps the design's length unit, so it no longer changes when the design's units change. Fixes #716.

crates/engine/src/lib.rs

@@ -803,6 +803,8 @@ mod construct_tests;
 #[cfg(test)]
 mod delete_face_tests;
 #[cfg(test)]
+mod derived_units_tests;
+#[cfg(test)]
 mod display_mesh_tests;
 #[cfg(test)]
 mod extent_tests;