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/build_web.py

@@ -149,6 +149,7 @@ def main():
     (web / "demo.json").write_text(json.dumps(demo_files(), ensure_ascii=False), encoding="utf-8")
     (web / "house.json").write_text(json.dumps(demo_files("house"), ensure_ascii=False), encoding="utf-8")
     (web / "factory.json").write_text(json.dumps(demo_files("factory"), ensure_ascii=False), encoding="utf-8")
+    (web / "streets.json").write_text(json.dumps(demo_files("streets"), ensure_ascii=False), encoding="utf-8")
 
     h = hashlib.sha256()
     for p in sorted((out / "assets").rglob("*")):