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/engine/src/cmd/joints_tests.rs

@@ -1178,3 +1178,6 @@ fn a_new_joint_starts_inside_its_limits_and_says_so() {
     let r = run(&mut s, "joint.as_built", json!({"type": "cylindrical", "a": "A", "b": "B", "name": "Spindle", "limits": [[-30, 30], [0, 5]]}));
     assert!(r.get("warnings").is_none(), "{r}");
 }
+
+#[path = "joints_coupling_tests.rs"]
+mod coupling;