Joints: a link ratio between an angle and a distance is per degree (#1433)

A motion link's ratio is read in b's units per a's (mm per degree, degrees per mm) when an angle and a distance are linked; stored values stay per radian, so saved designs are unchanged. Follow-up to #1415.

Co-authored-by: SychPL <[email protected]>

crates/ui-egui/src/dialogs_assembly.rs

@@ -878,7 +878,8 @@ pub fn commands(app: &SolveApp, k: &Asm, inputs: &[SelInput]) -> Result<Vec<(Str
         Asm::MotionLink { a, b, ia, ib, ratio, offset } => {
             let js = &s.doc.assembly.joints;
             let (Some(ja), Some(jb)) = (js.get(*a), js.get(*b)) else { return Err("needs two joints".into()) };
-            // The offset as typed: the engine reads it in Joint 2's units (deg or mm).
+            // The offset as typed: the engine reads it in Joint 2's units (deg or mm), and the
+            // ratio in Joint 2's units per Joint 1's.
             let ratio = eval(ratio, ValueKind::Unitless)?;
             cmd("joint.motion_link", json!({"a": ja.id, "b": jb.id, "ia": ia, "ib": ib, "ratio": ratio, "offset": offset.trim()}))
         }