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-project/src/mcp.rs

@@ -27,7 +27,9 @@ meet, so for a plot boundary put each piece's ends exactly on the next piece's e
 Production lines (category Industry): a producer (props.rate per minute, product) puts goods out on its local +x \
 side; a conveyor (rot 0 runs towards +x, 90 +y, 180 -x, 270 -y; w = length, props.speed m/s) starting exactly \
 at that side carries them, and a conveyor whose start is at another's end carries on with them (corners join). \
-The gap between goods is speed / rate, so set the rate on the producer and the speeds on the belts.";
+The gap between goods is speed / rate, so set the rate on the producer and the speeds on the belts. \
+A robot_arm (props.reach, default 2 m) 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 beside it; place it 1.5 to 2 m past a belt's end.";
 
 fn tool(name: &str, description: &str, props: Value, required: &[&str]) -> Value {
     json!({ "name": name, "description": description,