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.

docs/ROADMAP.md

@@ -72,8 +72,8 @@ Each phase ends with tests, a Studio/viewer check in Edge, commit and push.
 ## Backlog (2026-10)
 
 1. ~~Web performance~~ (done, see below)
-2. Conveyors: placeable conveyors
-3. Producers with adjustable production speed
+2. ~~Conveyors~~ (done, see below)
+3. ~~Producers with adjustable production speed~~ (done, see below)
 4. Robotic arms
 5. Rounded streets: curved road segments
 6. Sidewalks along streets
@@ -97,6 +97,12 @@ the tags only); hover 22 → 15 ms per move (tooltip placed once per frame on it
 readout ten times a second); rotate 226 → 154 ms, edit 409 → 305 ms until painted (WASM built with
 opt-level 3: engine render 11.7 → 9.4 ms at full speed).
 
+Production lines: Producer (a press making cartons, crates, drums, sacks or parts at a set rate
+per minute, its ram going down once per item) and Conveyor (drawn the way it runs, belt width,
+height, rise, speed; corners join without overlap). A conveyor starting at a producer's output, or
+at another conveyor's end, carries those goods, spaced by speed / rate; tooltips show the numbers.
+`examples/factory`, Factory demo in the web Studio.
+
 ## Phase 2: richer editing
 
 - [x] Interior prefabs (see Build your own house); a building `rooms` prop is still open