Joints: gears, rack and pinion, and screws (#1524)

Motion links can carry a coupling (gear pair, rack and pinion, screw) whose ratio follows radii, picked round faces or pitch. Design format 23.

Co-authored-by: SychPL <SychPL@users.noreply.github.com>
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

crates/ui-egui/src/workspace.rs

@@ -16,6 +16,9 @@ const ASSEMBLE: &[&str] = &[
     "joint.rigid_group",
     "joint.drive",
     "joint.motion_link",
+    "joint.gear",
+    "joint.rack_pinion",
+    "joint.screw",
     "motion.study",
     "explode.create",
     "contact.enable_sets",