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.

examples/streets/isokit.toml

@@ -1,7 +1,7 @@
 # isokit streets demo: roads with sidewalks, a rounded corner, junctions and a roundabout.
 # Open it with `isokit studio examples/streets`.
 title = "isokit streets"
-note = "Streets drawn along their centre lines: curves carry them round corners, side streets open the sidewalks they join, and the roundabout's kerbs flare into its arms. Tap a street for its numbers. Drag to pan, Ctrl + wheel or pinch to zoom."
+note = "Streets drawn along their centre lines: curves carry them round corners, side streets open the sidewalks they join, and the roundabout's kerbs flare into its arms. Cars go round the block, a bus, motorbikes, cyclists and people along their routes. Tap a street for its numbers. Drag to pan, Ctrl + wheel or pinch to zoom."
 default_level = "site"
 default_rotation = 1
 prelude = []