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.

crates/isokit-core/src/render.rs

@@ -415,14 +415,19 @@ impl Ctx<'_> {
                 id,
                 two_sided,
             } => self.face(pts, mat, id.as_deref(), *two_sided),
-            Prim::Poly { polys, cls, id } => {
+            Prim::Poly {
+                polys,
+                cls,
+                id,
+                open,
+            } => {
                 if polys.is_empty() {
                     return;
                 }
                 let p = self.p;
                 self.path_open(cls, id.as_deref());
                 for q in polys {
-                    p.d(&mut self.out, q, false);
+                    p.d(&mut self.out, q, *open);
                 }
                 self.out.push_str("\"/>");
             }