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/SCRIPTING.md

@@ -83,6 +83,7 @@ shaded automatically by face direction.
 | `cyl(cx, cy, r, z0, z1, mat[, opts])`, `frust(cx, cy, r0, r1, z0, z1, mat[, sides[, opts]])` | round things |
 | `slope(x0, x1, y0, y1, za, zb, th, mat)` | ramp / stair flight |
 | `face(pts3, mat[, #{id, two_sided}])` | any planar polygon, shaded and culled by its normal (roof slopes, gables) |
+| `beam(a, b, n, w, mat)` | square bar `w` wide between 3D points a and b, `n` a direction across it (robot arms, trusses) |
 | `poly(polys3, cls[, id])` | 3D polygons as one path (solar modules) |
 | `flat(poly or polys, z, cls[, id])` | flat polygon(s); later ones cut holes |
 | `open(a, b, [[u0, u1, z0, z1], …], cls[, id])` | windows / doors on the face a→b (outward normal on the right) |
@@ -122,8 +123,8 @@ cobble, brick) and the site `wall` use it; `house_wall` has its own version
 
 ## Production lines
 
-`producer` and `conveyor` (stdlib/industry.rhai) work together through the
-document, like house parts:
+`producer`, `conveyor` and `robot_arm` (stdlib/industry.rhai) work together
+through the document, like house parts:
 
 - A producer makes one of its `product` goods every 60 / `rate` seconds (its
   ram goes down once per item) and puts them out on its local +x side.
@@ -140,6 +141,17 @@ document, like house parts:
 - The goods ride a group that moves one gap per loop (`iso-ride`), so their
   drawing order never changes; the first fades in, the last fades out.
   Tooltips show speed, rate and where the goods come from (`info_row`).
+- A robot arm picks goods off the end of the nearest conveyor within `reach`
+  (+0.4 m) 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 of the arm). It
+  makes one move per item coming in (`cycle` 0) or every `cycle` seconds, and
+  waits while the line before it stands. The belt it puts on carries its goods
+  and starts its loop when they land, so arm and belts stay in step.
+- The arm is a flipbook: 16 poses (two-link inverse kinematics, links drawn
+  with `beam`), each shown for a sixteenth of the cycle by a stepped CSS
+  animation (`iso-flip`); a rotation cannot turn a 3D arm in this projection.
+  Poses pointing away from the viewer are drawn before the base, the others
+  after it.
 
 ## House parts