Conveyors and producers: goods flow from presses down the belts

- Producer (category Industry): a press making cartons, crates, drums, sacks or parts at a set
  rate per minute; its ram goes down once per item, a green / red light shows running / stopped.
- Conveyor: drawn the way it runs (new prefab meta `directed`: the Studio turns the piece from
  where the drag starts), belt width, height, rise (inclines), speed, goods and spacing; legs,
  rails and a drive motor. Corners join without overlap (the receiving belt runs through, the
  feeding one stops at its side).
- A conveyor starting on a producer's output side carries its goods at its rate; one starting at
  another's end carries on with what that one carries. The gap is speed / rate. A stopped press or
  belt stops the goods after it. Tooltips show speed, rate and source.
- The goods ride a group moving one gap per loop (no reordering), first fading in, last out.
- Script API: world_of(o, u, v), info_row(key, value); objects_on falls back to half of p.d.
- Theme: industry colours, LED colours (the marker's LED had none).
- examples/factory + Factory demo in the web Studio; MCP instructions on production lines.

Checks: 68 Rust tests (conveyor joints, feed and spacing, every view of the factory demo),
Studio 108/108 (draw direction, feed from a producer, rate change), web 89/89.

tools/build_web.py

@@ -148,6 +148,7 @@ def main():
         shutil.copy2(ROOT / src, out / "assets" / dst / Path(src).name)
     (web / "demo.json").write_text(json.dumps(demo_files(), ensure_ascii=False), encoding="utf-8")
     (web / "house.json").write_text(json.dumps(demo_files("house"), ensure_ascii=False), encoding="utf-8")
+    (web / "factory.json").write_text(json.dumps(demo_files("factory"), ensure_ascii=False), encoding="utf-8")
 
     h = hashlib.sha256()
     for p in sorted((out / "assets").rglob("*")):