Streets: sidewalks, road curves, junctions and roundabouts

Roads take sidewalks (none, both, left, right): 12 cm high paving with
kerb faces drawn where they face the viewer. Road curve (radius, angle,
left/right) carries a street round a bend; centre lines are true arcs
when cut and offset, so curve joints are seamless. A road ending on
another road's centre line opens that road's kerb and sidewalk (T
junction); two roads ending at one corner close it. Roundabout: ring
radius and width, one or two lanes, up to four arms with flared kerbs,
splitter islands, give-way lines on the entry side (traffic on the left
or right) and a central island with apron.

Studio: a road run drawn corner by corner gets a curve in each corner
(prefab meta `corner`). Tooltips list the streets each one meets.
Engine: open `polyline` primitive (Prim::Poly open) so dashes run on
round curves. examples/streets and a Streets demo in the web Studio;
MCP guidance, docs. ROADMAP: backlog 5-7 done; multi-select and the
hosted copy marked done.

Tests: Rust 72, Studio 113, web 95.

tools/check_studio.py

@@ -471,6 +471,38 @@ with sync_playwright() as p:
     pg.evaluate("__studio.select(null)")
     pg.wait_for_timeout(300)
 
+    # a road run drawn corner by corner off the main road: a curve rounds its corner, the road it
+    # starts on gets a junction (its south kerb opens), and one undo step takes the run back
+    pg.evaluate("__studio.view.focus([6, -27, 0], 2.2)")
+    pg.wait_for_timeout(400)
+    n4 = len(doc()["objects"])
+    kerbs = lambda: pg.locator('.iso-svg g[data-obj="road"] .iso-kerb').count()
+    k0 = kerbs()
+    pg.locator('.st-tile[data-type="road"]').click()
+    for x, y in [(-2, -19.3), (-2.1, -32), (20, -32.1)]:
+        pg.mouse.click(*at(x, y))
+        pg.wait_for_timeout(150)
+    pg.mouse.move(*at(20, -36))
+    pg.wait_for_timeout(150)
+    check("corners rounded" in pg.locator("#hint").inner_text(), "a road run says its corners get rounded: " + pg.locator("#hint").inner_text()[:50])
+    pg.keyboard.press("Enter")
+    pg.wait_for_timeout(1200)
+    run = doc()["objects"][n4:]
+    got = sorted((o["type"], o["x"], o["y"], o.get("rot", 0), o["props"].get("w", o["props"].get("r"))) for o in run)
+    want = sorted([("road", -2, -20.75, 90, 2.5), ("road", 14, -32, 0, 12), ("road_curve", -2, -22, 270, 10)])
+    check(got == want, f"the run's corner gets a curve: {got}")
+    rv = get("/api/render?level=site&rot=0")
+    stub = next((o["id"] for o in run if o["type"] == "road" and o["x"] == -2), None)
+    sb = rv["boxes"].get(stub, [0] * 6)
+    check(abs(sb[4] + 19.5) < 1e-6 and kerbs() == k0 + 1,
+          f"the run starts on the main road's centre line and opens its kerb: {sb[4]:.2f}, {k0} -> {kerbs()} kerb lines")
+    pg.evaluate("__studio.select(null)")
+    pg.wait_for_timeout(300)
+    pg.screenshot(path=str(OUT / "studio-road-run.jpg"), type="jpeg", quality=80)
+    pg.locator("#undo").click()
+    pg.wait_for_timeout(900)
+    check(len(doc()["objects"]) == n4 and kerbs() == k0, "one undo takes the road run back")
+
     # an agent edits over MCP: the open Studio follows
     init = mcp({"jsonrpc": "2.0", "id": 1, "method": "initialize", "params": {"protocolVersion": "2025-06-18", "capabilities": {}, "clientInfo": {"name": "test", "version": "0"}}})
     check(init["result"]["serverInfo"]["name"] == "isokit", "MCP initialize")