Routes: movers along waypoints (roadmap 8)

New route prefab (routes.rhai): cars, buses, motorbikes, cyclists, people,
deer or elephants move along waypoints, round a loop or there and back.
Each way keeps to the left of the line (or right), corners are rounded
into pieces of at most 45 degrees; there-and-back routes become loops with
U-turns, so the two directions never share a strip.

Engine: Prim::Travel / sprite.begin_travel / script travel(a, b, dur,
windows) ... end(): the mover is drawn at a piece's start and slides to
its end while it is on that piece, hidden otherwise. The renderer writes
keyframes per piece (named by a hash of their content) into one <style>
per view. Each piece is its own sprite with the lane strip as its sort
extent (ext_reset), mitred at the corners, so movers pass in front of and
behind trees, lamps and houses. Movers that look alike and never meet on
a piece share one drawing. iso-travel pauses (not hides) in still,
reduced-motion and ghosted views.

Studio: meta shape "path" draws point by point (snapping like runs) and
saves one object with pts; ending on the first point closes the loop.
MCP: pts in the object schema, route guidance.

Demos: streets gets traffic round the block, a bus, motorbikes, cyclists
and strolling people (replacing the one-axis cars); country gets cars on
the lane.

Tests: Rust 79 (each mover on exactly one piece at every moment, in every
rotation), Studio 125, web 99.

docs/ROADMAP.md

@@ -80,7 +80,7 @@ Each phase ends with tests, a Studio/viewer check in Edge, commit and push.
 5. ~~Rounded streets: curved road segments~~ (done, see below)
 6. ~~Sidewalks along streets~~ (done, see below)
 7. ~~Roundabouts~~ (done, see below)
-8. Waypoint animation: a person or car moving along a path of waypoints
+8. ~~Waypoint animation: a person or car moving along a path of waypoints~~ (done, see below)
 9. ~~Build your own house~~ (done, see above)
 10. ~~Diagonal streets, walls, fences, hedges~~ (done, see below)
 11. ~~Wildlife and nature: deer, bushes, beehives and more~~ (done, see below)
@@ -145,6 +145,13 @@ turns while grazing, rabbits hopping; Birds and Butterflies as flocks (`swim` sh
 out of step); Beehives with bees, Bush, Flowers (bed or meadow), Rocks, Log / stump.
 `examples/country`, Country demo in the web Studio.
 
+Routes (`routes.rhai`, `travel` primitive): cars, buses, motorbikes, cyclists, people, deer and
+elephants along waypoints drawn point by point in the Studio (`shape: "path"`, `pts`); loops or
+there and back, each way on its side of the line, rounded corners. Every straight piece is a
+sprite whose sort box is its lane strip, so movers pass in front of and behind what stands
+beside the road; keyframes per piece show a mover only while it is on it. Streets demo: traffic
+round the block, a bus, motorbikes, cyclists, people; country demo: cars on the lane.
+
 ## Phase 2: richer editing
 
 - [x] Interior prefabs (see Build your own house); a building `rooms` prop is still open