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/country/scene.json

@@ -39,6 +39,7 @@
     {"id": "bush-5", "type": "bush", "x": -36, "y": -18, "rot": 0, "props": {"r": 0.8, "kind": "flowering", "colour": "pink"}},
     {"id": "pond", "type": "pond", "name": "Pond", "x": 36, "y": -6, "rot": 0, "props": {"w": 11, "d": 8, "fish": 6, "lilies": 4}},
     {"id": "pond-rocks", "type": "rocks", "x": 42, "y": -1, "rot": 0, "props": {"n": 2, "r": 0.6, "spread": 1}},
-    {"id": "birds", "type": "birds", "name": "Birds", "x": 0, "y": 0, "rot": 0, "props": {"n": 10, "h": 18, "r": 14}}
+    {"id": "birds", "type": "birds", "name": "Birds", "x": 0, "y": 0, "rot": 0, "props": {"n": 10, "h": 18, "r": 14}},
+    {"id": "lane-cars", "type": "route", "name": "Cars on the lane", "x": -14.0, "y": 2.5, "pts": [[-32.0, -32.5], [2.0, -32.5], [32.0, -2.5], [32.0, 32.5]], "props": {"kind": "car", "n": 2, "path": false, "colour": "mixed"}}
   ]
 }