Point handles for routes and outlines; Area corners; bus goes round right

Studio: one selected object with pts (a route, an outlined area) shows a
handle on each point and a + in the middle of each side: drag a point to
move it, drag a + to add one there, double-click a point to remove it
(open routes keep 2, outlines 3). Points snap to the grid and onto wall,
fence and road ends, at any angle; one undo step each. Handles keep their
screen size (new viewer callback onView on every redraw) and follow
re-renders and selection. The viewer captures the pointer, so a double-
click targets the drawing: the last point pressed is remembered instead.

Prefab meta `outline`: the prefab draws pts as its outline. Area has it:
Edit corners turns its rectangle into four points, Rectangle again
removes them.

Routes: where a path doubles back, the lane offset puts a point on each
side, so it rounds into a U-turn instead of a diagonal across the lanes.
The streets demo bus now runs east, three quarters round the roundabout
(clockwise, left-hand traffic), down south street, turns and comes back
by the short way, instead of going round the wrong way on its way out.

Tests: Rust 81 (a doubling-back route: each mover once, no sort cycles),
Studio 134 (corners, move, add, remove, undo), web 100.

crates/isokit-script/src/tests.rs

@@ -936,8 +936,24 @@ fn route_movers_show_once_each_at_every_moment() {
     );
     deer.rot = 30.0;
     deer.pts = Some(vec![[-6.0, 0.0], [6.0, 0.0]]);
+    // a loop that doubles back twice (a spur up to a stop and back, a U-turn at the west end):
+    // each way keeps to its side, so the strips never overlap
+    let mut spur = obj(
+        "spur",
+        "route",
+        0.0,
+        40.0,
+        json!({"kind": "bus", "n": 2, "closed": true}),
+    );
+    spur.pts = Some(vec![[-20.0, 0.0], [20.0, 0.0], [20.0, 15.0], [20.0, 0.0]]);
     let doc = Doc {
-        objects: vec![cars, walk, deer, obj("t1", "tree", 0.0, -14.0, json!({}))],
+        objects: vec![
+            cars,
+            walk,
+            deer,
+            spur,
+            obj("t1", "tree", 0.0, -14.0, json!({})),
+        ],
         ..Default::default()
     };
     for view in 0..4u8 {
@@ -947,7 +963,7 @@ fn route_movers_show_once_each_at_every_moment() {
         assert_eq!(out.cycles, 0, "view {view}: depth-sort cycles");
         // one <style> with every keyframe set, each used
         assert_eq!(out.html.matches("<style>").count(), 1);
-        for (id, n) in [("cars", 4), ("walk", 3), ("deer", 2)] {
+        for (id, n) in [("cars", 4), ("walk", 3), ("deer", 2), ("spur", 2)] {
             let wins = travel_windows(&b, id);
             assert!(!wins.is_empty(), "{id}");
             // at any moment, each mover is on exactly one piece