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.

docs/ROADMAP.md

@@ -74,7 +74,7 @@ Each phase ends with tests, a Studio/viewer check in Edge, commit and push.
 1. ~~Web performance~~ (done, see below)
 2. ~~Conveyors~~ (done, see below)
 3. ~~Producers with adjustable production speed~~ (done, see below)
-4. Robotic arms
+4. ~~Robotic arms~~ (done, see below)
 5. Rounded streets: curved road segments
 6. Sidewalks along streets
 7. Roundabouts
@@ -103,6 +103,11 @@ height, rise, speed; corners join without overlap). A conveyor starting at a pro
 at another conveyor's end, carries those goods, spaced by speed / rate; tooltips show the numbers.
 `examples/factory`, Factory demo in the web Studio.
 
+Robot arm: picks goods off the end of the nearest belt in its reach and puts them on the start of
+another belt or on a pallet, one move per item (or a set cycle), waiting while the line before it
+stands; the belt after it runs in step. Drawn as a 16-pose flipbook (inverse kinematics, links as
+the new native `beam` primitive) shown by a stepped CSS animation.
+
 ## Phase 2: richer editing
 
 - [x] Interior prefabs (see Build your own house); a building `rooms` prop is still open