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.

README.md

@@ -90,6 +90,15 @@ trunks, rabbits hop, birds flap. `examples/country` (Country demo in the web
 Studio) puts them in a countryside corner with a lane, fence and hedgerow at
 an angle.
 
+**Routes**: a Route moves cars, buses, motorbikes, cyclists, people, deer or
+elephants along waypoints. Pick Route and click point after point on the map
+(points snap onto road and wall ends; double-click or Enter ends it, the
+first point again makes a loop). A loop is driven round, an open route there
+and back; each way keeps to the left of the line (or the right), corners are
+rounded, and the movers pass in front of and behind trees and houses as they
+go. The streets demo has traffic round the block, a bus, motorbikes, cyclists
+and people strolling.
+
 **Fences and walls** around the plot: the Fence has six designs (mesh, wooden
 boards, steel bars, mesh on a stone base, cobble wall, brick wall with
 pillars). Fences and site walls whose ends meet join like house walls: corners