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/tests.rs

@@ -643,6 +643,95 @@ fn conveyors_carry_what_the_producer_makes() {
     assert_eq!(row("belt3", "Speed"), "0.8 m/s");
 }
 
+fn robot_doc(belt1_running: bool) -> Doc {
+    let mut doc = Doc::default();
+    for (id, kind, x, y, rot, props) in [
+        ("press1", "producer", -6.0, -4.0, 0.0, json!({"rate": 15})),
+        (
+            "belt1",
+            "conveyor",
+            -1.4,
+            -4.0,
+            0.0,
+            json!({"w": 6, "running": belt1_running}),
+        ),
+        ("robot1", "robot_arm", 3.2, -4.0, 0.0, json!({})),
+        ("belt2", "conveyor", 3.2, 1.0, 90.0, json!({"w": 6})),
+        (
+            "robot2",
+            "robot_arm",
+            3.2,
+            5.8,
+            0.0,
+            json!({"colour": "mach2"}),
+        ),
+        (
+            "robot3",
+            "robot_arm",
+            -6.0,
+            5.0,
+            0.0,
+            json!({"running": false, "pick": "+x", "put": "-y", "goods": "drum"}),
+        ),
+    ] {
+        let mut o = obj(id, kind, x, y, props);
+        o.rot = rot;
+        doc.objects.push(o);
+    }
+    doc
+}
+
+#[test]
+fn robot_arms_pass_goods_from_belt_to_belt() {
+    let r = Runtime::new(&Bundle::default()).unwrap();
+    let doc = robot_doc(true);
+    for view in 0..4u8 {
+        let b = r.build("site", view, &cfg(), &doc).unwrap();
+        assert!(b.scene.warnings.is_empty(), "{:?}", b.scene.warnings);
+        let out = render(&b.scene, view, &RenderOpts::default());
+        assert_eq!(out.cycles, 0, "view {view}: depth-sort cycles");
+        assert!(!out.html.contains("NaN"));
+        // two running arms with 16 poses each, one shown at a time; the stopped one stands still
+        assert_eq!(
+            out.html.matches("iso-a iso-flip").count(),
+            32,
+            "view {view}"
+        );
+        assert_eq!(
+            out.html.matches("iso-flip iso-pose0").count(),
+            2,
+            "view {view}"
+        );
+    }
+    let b = r.build("site", 1, &cfg(), &doc).unwrap();
+    let out = render(&b.scene, 1, &RenderOpts::default());
+    let row = |id: &str, k: &str| {
+        out.info[id]
+            .rows
+            .iter()
+            .find(|r| r.0 == k)
+            .map(|r| r.1.clone())
+            .unwrap_or_default()
+    };
+    assert_eq!(row("robot1", "Moves"), "cartons from belt1 to belt2");
+    assert_eq!(row("robot1", "Cycle"), "4 s");
+    // the belt after the arm carries its goods at its pace, starting its loop when they land
+    assert_eq!(
+        row("belt2", "Carries"),
+        "15 cartons a minute, 2 m apart (from robot1)"
+    );
+    assert!(out
+        .html
+        .contains("animation-duration:4.0s;animation-delay:-1.75s"));
+    assert_eq!(row("robot2", "Moves"), "cartons from belt2 to a pallet");
+    assert_eq!(row("robot3", "Cycle"), "4 s (stopped)");
+    // a stopped belt before it: the arm waits
+    let b = r.build("site", 1, &cfg(), &robot_doc(false)).unwrap();
+    let out = render(&b.scene, 1, &RenderOpts::default());
+    assert_eq!(out.html.matches("iso-a iso-flip").count(), 0);
+    assert!(out.info["robot1"].rows[1].1.contains("waiting"));
+}
+
 #[test]
 fn site_walls_join_at_corners() {
     let r = Runtime::new(&Bundle::default()).unwrap();