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.

examples/factory/isokit.toml

@@ -1,7 +1,8 @@
-# isokit factory demo: producers making goods at a set rate, conveyors carrying them.
+# isokit factory demo: producers making goods at a set rate, conveyors carrying them,
+# robot arms passing them on.
 # Open it with `isokit studio examples/factory`.
 title = "isokit factory"
-note = "Presses make goods at a set rate; conveyors carry them at their own speed, so the gap between the goods follows rate and speed. Tap a press or a belt for its numbers. Drag to pan, Ctrl + wheel or pinch to zoom."
+note = "Presses make goods at a set rate; conveyors carry them at their own speed, so the gap between the goods follows rate and speed; robot arms pass them on. Tap a press, belt or robot for its numbers. Drag to pan, Ctrl + wheel or pinch to zoom."
 default_level = "site"
 default_rotation = 1
 prelude = []