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/check_studio.py

@@ -345,6 +345,48 @@ with sync_playwright() as p:
     pg.wait_for_timeout(600)
     pg.evaluate("__studio.select(null)")
 
+    # a conveyor runs the way it was drawn; started at a producer's output it carries what the producer
+    # makes, spaced by its rate (changed in the producer's properties) and the belt's speed
+    pg.evaluate("__studio.select(null)")
+    pg.evaluate("__studio.view.focus([25, -4.5, 0], 3)")
+    pg.wait_for_timeout(400)
+    n4 = len(doc()["objects"])
+    pg.locator('.st-tile[data-type="conveyor"]').click()
+    a, b2 = at(27, -4.5), at(21, -4.5)
+    pg.mouse.move(*a)
+    pg.mouse.down()
+    for i in range(1, 11):
+        pg.mouse.move(a[0] + (b2[0] - a[0]) * i / 10, a[1] + (b2[1] - a[1]) * i / 10)
+        pg.wait_for_timeout(15)
+    check("running from where you started" in pg.locator("#hint").inner_text(), "the conveyor preview says which way it runs")
+    pg.mouse.up()
+    pg.wait_for_timeout(800)
+    cv = doc()["objects"][-1]
+    check(cv["type"] == "conveyor" and cv.get("rot") == 180 and abs(cv["x"] - 24) < 1e-6 and abs(cv["props"]["w"] - 6) < 1e-6,
+          f"a conveyor dragged right to left runs that way (rot {cv.get('rot')}, x {cv['x']})")
+    pg.evaluate("__studio.op({op: 'add', type: 'producer', x: 28.6, y: -4.5, rot: 180, props: {rate: 20}})")
+    pg.wait_for_timeout(900)
+    pid = doc()["objects"][-1]["id"]
+    rows = lambda: dict(pg.evaluate(f"(__studio.view.info('{cv['id']}') || {{}}).rows || []"))
+    check(f"(from {pid})" in rows().get("Carries", "") and "20 cartons a minute, 1.5 m apart" in rows().get("Carries", ""),
+          "the belt carries what the producer at its start makes: " + rows().get("Carries", "?"))
+    check(pg.evaluate(f"document.querySelectorAll('.iso-svg [data-obj=\"{cv['id']}\"] .iso-ride').length") == 1 and
+          pg.evaluate(f"document.querySelectorAll('.iso-svg [data-obj=\"{pid}\"] .iso-ram').length") == 1,
+          "goods ride the belt, the press's ram goes up and down")
+    pg.locator(f'#objects li[data-id="{pid}"]').click()
+    pg.wait_for_timeout(300)
+    pg.locator('#right input[type="number"][data-prop="rate"]').fill("30")
+    pg.locator('#right input[type="number"][data-prop="rate"]').press("Enter")
+    pg.wait_for_timeout(900)
+    check("30 cartons a minute, 1 m apart" in rows().get("Carries", ""), "a faster producer packs the belt closer: " + rows().get("Carries", "?"))
+    pg.screenshot(path=str(OUT / "studio-conveyor.jpg"), type="jpeg", quality=80)
+    for _ in range(3):
+        pg.locator("#undo").click()
+        pg.wait_for_timeout(500)
+    check(len(doc()["objects"]) == n4, "undo takes the producer and the belt back")
+    pg.evaluate("__studio.select(null)")
+    pg.wait_for_timeout(300)
+
     # performance: animations step about ten times a second by default (no CSS animation runs on its
     # own), the toolbar button switches smooth / off / light; a pan moves the drawing as one layer and
     # draws it again (viewBox) only when the drag ends