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.

web/web.js

@@ -367,7 +367,7 @@ function injectUI() {
     '<h3>Projects</h3>' +
     '<div class="wb-cur" id="wb-cur"></div>' +
     '<ul class="wb-plist" id="wb-plist"></ul>' +
-    '<div class="wb-new"><input id="wb-ntitle" placeholder="Title of a new project" autocomplete="off"><button type="button" id="wb-ncreate">New project</button><button type="button" id="wb-ndemo" title="A fresh copy of the demo scene">Demo copy</button><button type="button" id="wb-nhouse" title="A house built from parts: walls, windows, doors, floors, furniture">House demo</button><button type="button" id="wb-nfactory" title="Presses making goods at a set rate, conveyors carrying them">Factory demo</button></div>' +
+    '<div class="wb-new"><input id="wb-ntitle" placeholder="Title of a new project" autocomplete="off"><button type="button" id="wb-ncreate">New project</button><button type="button" id="wb-ndemo" title="A fresh copy of the demo scene">Demo copy</button><button type="button" id="wb-nhouse" title="A house built from parts: walls, windows, doors, floors, furniture">House demo</button><button type="button" id="wb-nfactory" title="Presses making goods at a set rate, conveyors carrying them">Factory demo</button><button type="button" id="wb-nstreets" title="Streets with sidewalks, a rounded corner, junctions and a roundabout">Streets demo</button></div>' +
     '<div class="wb-drop" id="wb-drop">Import: drop a project folder or .zip here, or <button type="button" id="wb-izip" title="Import a project .zip into this browser">.zip…</button><button type="button" id="wb-idir" title="Copy a project folder into this browser">folder…</button>' +
     (canOpenFolder ? '<span class="st-grow"></span><button type="button" id="wb-ofolder" title="Edit the files in the folder on disk directly; changes made there show up live">Open folder live…</button>' : "") + "</div>" +
     '<p class="st-help">Projects are kept in this browser. ' + (canOpenFolder ? "A folder opened live is edited on disk, so agents (<b>isokit mcp</b>), editors and git see the same files." : "To edit a folder on disk directly, use Chrome or Edge.") + "</p>" +
@@ -419,6 +419,7 @@ function wireProjects() {
   $("wb-ndemo").addEventListener("click", async function () { openProject((await createProject(await demoFiles())).id); });
   $("wb-nhouse").addEventListener("click", async function () { openProject((await createProject(await demoFiles("house.json"))).id); });
   $("wb-nfactory").addEventListener("click", async function () { openProject((await createProject(await demoFiles("factory.json"))).id); });
+  $("wb-nstreets").addEventListener("click", async function () { openProject((await createProject(await demoFiles("streets.json"))).id); });
   $("wb-izip").addEventListener("click", function () { $("wb-fzip").click(); });
   $("wb-idir").addEventListener("click", function () { $("wb-fdir").click(); });
   $("wb-fzip").addEventListener("change", function () { var f = this.files[0]; if (f) importZip(f); this.value = ""; });