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.

viewer/isokit-view.css

@@ -140,10 +140,14 @@ html[data-theme=light] .iso-svg .iso-glint { display: inline; }
 .iso-svg .iso-flutter { transform-box: fill-box; transform-origin: center; animation: iso-flap .3s ease-in-out infinite alternate; }
 .iso-svg .iso-hop { animation: iso-hop 3.4s ease-out infinite; }
 .iso-svg .iso-park { display: none; }
+/* route movers: each piece's copy shows only while a mover is on it (keyframes made per piece);
+   not .iso-a, so unanimated views pause them where they are instead of hiding them */
+.iso-svg .iso-travel { visibility: hidden; }
 /* paused off-screen (iso-off), when stopped (iso-still), with animation off and during pan/zoom gestures */
 .iso-off .iso-a, .iso-still .iso-a, .iso-off .iso-drive, .iso-still .iso-drive, .iso-off .iso-ledr, .iso-off .iso-ledo, .iso-still .iso-ledr, .iso-still .iso-ledo,
 .iso-anim-off .iso-a, .iso-anim-off .iso-drive, .iso-anim-off .iso-ledr, .iso-anim-off .iso-ledo,
-.iso-gesture .iso-a, .iso-gesture .iso-drive, .iso-gesture .iso-ledr, .iso-gesture .iso-ledo { animation-play-state: paused !important; }
+.iso-gesture .iso-a, .iso-gesture .iso-drive, .iso-gesture .iso-ledr, .iso-gesture .iso-ledo,
+.iso-off .iso-travel, .iso-still .iso-travel, .iso-anim-off .iso-travel, .iso-gesture .iso-travel, .iso-svg .iso-dim .iso-travel { animation-play-state: paused !important; }
 .iso-still .iso-park, .iso-anim-off .iso-park { display: inline; }
 .iso-animbtn { min-width: 30px; font-size: 13px; }
 .iso-bar [hidden] { display: none !important; }
@@ -180,6 +184,7 @@ html[data-theme=light] .iso-svg .iso-glint { display: inline; }
 /* a system asking for less motion: still pictures, until the viewer turns animation on (iso-anim-on) */
 @media (prefers-reduced-motion: reduce) {
   .iso-wrap:not(.iso-anim-on) .iso-svg .iso-a, .iso-wrap:not(.iso-anim-on) .iso-svg .iso-ledr, .iso-wrap:not(.iso-anim-on) .iso-svg .iso-ledo { animation: none !important; }
+  .iso-wrap:not(.iso-anim-on) .iso-svg .iso-travel { animation-play-state: paused !important; }
   .iso-wrap:not(.iso-anim-on) .iso-svg :is(.iso-puff, .iso-puff2, .iso-smoke, .iso-disc, .iso-glint, .iso-rip) { display: none; }
   .iso-wrap:not(.iso-anim-on) .iso-svg .iso-park { display: inline; }
 }