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-script/src/api.rs

@@ -297,6 +297,14 @@ pub fn register_sprite(e: &mut Engine) {
             ok(s)
         },
     );
+    e.register_fn(
+        "beam",
+        |s: &mut SpriteH, a: D, b: D, nn: D, w: D, mat: &str| -> RR<SpriteH> {
+            s.0.borrow_mut()
+                .beam(p3(&a)?, p3(&b)?, p3(&nn)?, n(&w)?, &classes(mat));
+            ok(s)
+        },
+    );
     e.register_fn("face", |s: &mut SpriteH, p: D, mat: &str| -> RR<SpriteH> {
         s.0.borrow_mut().face(pts3(&p)?, &classes(mat), None, false);
         ok(s)