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

examples/factory/scene.json

@@ -0,0 +1,30 @@
+{
+  "version": 1,
+  "objects": [
+    {"id": "site", "type": "site", "x": 0, "y": 1, "props": {"w": 46, "d": 32, "mat": "g-grass"}, "name": "Yard", "locked": true},
+    {"id": "road", "type": "road", "x": 0, "y": -12.5, "props": {"w": 46, "d": 5}, "name": "Access road"},
+    {"id": "floor", "type": "area", "x": 0, "y": 1.5, "props": {"w": 36, "d": 19, "mat": "g-conc"}, "name": "Production floor", "locked": true},
+    {"id": "press-a", "type": "producer", "x": -14, "y": -4, "props": {"rate": 20, "product": "carton", "colour": "mach1"}, "name": "Carton press", "tag": "Cartons 20/min"},
+    {"id": "belt-a1", "type": "conveyor", "x": -8.4, "y": -4, "rot": 0, "props": {"w": 8}, "name": "Carton belt 1"},
+    {"id": "belt-a2", "type": "conveyor", "x": -4.4, "y": -1, "rot": 90, "props": {"w": 6}, "name": "Carton belt 2"},
+    {"id": "belt-a3", "type": "conveyor", "x": -0.4, "y": 2, "rot": 0, "props": {"w": 8, "speed": 0.8}, "name": "Carton belt 3"},
+    {"id": "press-b", "type": "producer", "x": 14, "y": -4, "rot": 180, "props": {"rate": 10, "product": "drum", "colour": "mach2", "d": 2.6}, "name": "Drum filler", "tag": "Drums 10/min"},
+    {"id": "belt-b1", "type": "conveyor", "x": 9.4, "y": -4, "rot": 180, "props": {"w": 6, "d": 1.0, "speed": 0.4}, "name": "Drum belt"},
+    {"id": "belt-b2", "type": "conveyor", "x": 6.4, "y": 0, "rot": 90, "props": {"w": 8, "d": 1.0, "speed": 0.4, "rise": 1.4}, "name": "Drum incline"},
+    {"id": "press-c", "type": "producer", "x": -14, "y": 6, "props": {"rate": 6, "product": "part", "colour": "mach4", "running": false}, "name": "Parts press", "tag": "Stopped", "info": "Waiting for a tool change"},
+    {"id": "belt-c1", "type": "conveyor", "x": -9.9, "y": 6, "rot": 0, "props": {"w": 5}, "name": "Parts belt"},
+    {"id": "belt-load", "type": "conveyor", "x": 6.5, "y": 8.5, "rot": 0, "props": {"w": 7, "item": "sack", "gap": 1.1, "speed": 0.6, "rise": 1.0}, "name": "Loading belt"},
+    {"id": "truck", "type": "truck", "x": 13.2, "y": 8.5, "rot": 90, "props": {"len": 9}, "name": "Truck at the dock"},
+    {"id": "office", "type": "shed", "x": -5.5, "y": 9.2, "props": {"w": 6, "d": 3.5, "h": 3}, "name": "Shift office"},
+    {"id": "cont-1", "type": "container", "x": 16.8, "y": 0.2, "rot": 90, "props": {"size": "20ft", "stack": 2}, "name": "Empty drums"},
+    {"id": "cont-2", "type": "container", "x": -16.6, "y": 1.5, "rot": 90, "props": {"size": "20ft", "colour": "truckbox"}, "name": "Carton blanks"},
+    {"id": "op-a", "type": "person", "x": -14.2, "y": -6.2},
+    {"id": "op-c", "type": "person", "x": -11.8, "y": 7.8, "props": {"cls": "hivis"}},
+    {"id": "op-load", "type": "person", "x": 2.4, "y": 9.6},
+    {"id": "tree-1", "type": "tree", "x": -20, "y": 13},
+    {"id": "tree-2", "type": "tree", "x": -15, "y": 14},
+    {"id": "tree-3", "type": "tree", "x": 19, "y": 14},
+    {"id": "lamp-1", "type": "lamp", "x": -10, "y": -9.6},
+    {"id": "lamp-2", "type": "lamp", "x": 10, "y": -9.6}
+  ]
+}