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.

tools/check_web.py

@@ -340,6 +340,13 @@ try:
         meets = pg.evaluate("() => Object.fromEntries((__studio.view.info('cross-st') || {rows: []}).rows)['Meets'] || ''")
         ring = pg.evaluate("() => Object.fromEntries((__studio.view.info('rb') || {rows: []}).rows)['Arms'] || ''")
         check(meets == "east-ave, park-rd" and ring.startswith("4:"), f"streets know what they meet: cross-st meets {meets}; roundabout arms {ring}")
+        # route movers: each shows once at any moment (cars 4, bus 1, motorbikes 3, cyclists 2, people 3)
+        movers = pg.evaluate("""() => { const out = {}; for (const id of ['traffic', 'bus-line', 'motos', 'cyclists', 'strollers'])
+            out[id] = [...document.querySelectorAll(`.iso-svg [data-obj='${id}'] .iso-travel`)].filter(e => getComputedStyle(e).visibility === 'visible').length;
+            return out; }""")
+        traffic = pg.evaluate("() => Object.fromEntries((__studio.view.info('traffic') || {rows: []}).rows)['Movers'] || ''")
+        check(movers == {"traffic": 4, "bus-line": 1, "motos": 3, "cyclists": 2, "strollers": 3} and traffic.startswith("4 cars, 8 m/s"),
+              f"movers on their routes, each once: {movers}; {traffic}")
         shot(pg, "web-streets.jpg")
 
         # ---------------------------------------------------------------- the country demo: animals, an apiary, diagonals