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.

examples/factory/isokit.toml

@@ -0,0 +1,29 @@
+# isokit factory demo: producers making goods at a set rate, conveyors carrying them.
+# 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."
+default_level = "site"
+default_rotation = 1
+prelude = []
+theme = []
+
+[[level]]
+id = "site"
+label = "Site"
+title = "The production yard"
+
+[[param]]
+name = "labels"
+type = "bool"
+default = true
+label = "Labels"
+
+[[param]]
+name = "running"
+type = "bool"
+default = true
+label = "Animation"
+
+[build]
+out = "dist/index.html"
+budget = 6000