Robot arms pass goods from belt to belt; native beam primitive

- Robot arm (category Industry): picks goods off the end of the nearest conveyor within its reach
  and puts them on the start of another conveyor within reach, or on a pallet a quarter turn on
  (pick / put: auto or a side). One move per item coming in (cycle 0) or a set cycle; waits while
  the line before it stands. The belt it feeds carries its goods at its pace and starts its loop
  when they land (feeds carry a time offset `ph`), so arm and belts stay in step.
- Drawn as a 16-pose flipbook: two-link inverse kinematics per pose, each pose shown for a
  sixteenth of the cycle by a stepped CSS animation (iso-flip; pose 0 alone when unanimated).
  Poses pointing away from the viewer are drawn before the base, the others after it.
- New sprite method beam(a, b, n, w, mat): a square bar between two 3D points, six faces culled
  and shaded like face(). Native, so the factory demo renders in 4.7 ms instead of 18.8 ms.
- Factory demo: a palletiser at the end of the carton line, a loading robot passing drums from the
  incline onto the loading belt into the truck. MCP instructions on robot arms.

Checks: 70 Rust tests (arms and belts in step, waiting arm, beam faces from every side),
Studio 109/109, web 90/90.

crates/isokit-core/src/render.rs

@@ -870,6 +870,35 @@ mod tests {
         }
     }
 
+    #[test]
+    fn beams_show_three_faces_from_any_side() {
+        for (a, b, n) in [
+            ([0.0, 0.0, 1.0], [4.0, 0.0, 1.0], [0.0, 1.0, 0.0]),
+            ([0.0, 0.0, 1.0], [0.0, -4.0, 1.0], [1.0, 0.0, 0.0]),
+            ([0.0, 0.0, 0.0], [0.0, 0.0, 3.0], [1.0, 0.0, 0.0]),
+            ([0.0, 0.0, 0.0], [2.0, 1.0, 3.0], [-1.0, 2.0, 0.0]),
+        ] {
+            for rot in 0..4u8 {
+                let mut sc = Scene::new();
+                let mut s = Sprite::new("arm", "");
+                s.beam(a, b, n, 0.4, "iso-robot");
+                sc.add_sprite("main", s);
+                let html = render(&sc, rot, &RenderOpts::default()).html;
+                // three faces; two when one side is seen edge-on (an oblique bar)
+                let faces = html.matches("iso-robot ").count();
+                let oblique = a[0] != b[0] && a[2] != b[2];
+                assert!(
+                    faces == 3 || (oblique && faces == 2),
+                    "{a:?}->{b:?} rot {rot}: {faces} faces"
+                );
+                assert!(
+                    html.contains("iso-robot iso-t"),
+                    "{a:?}->{b:?} rot {rot}: no top"
+                );
+            }
+        }
+    }
+
     #[test]
     fn faces_cull_and_shade_by_normal() {
         let mut sc = Scene::new();