Web performance: light animation, gestures on a compositor layer, no forced layouts

Measured with the new tools/bench_web.py on the web Studio's house demo, CPU slowed 6x:

- Idle: running CSS animations repainted the whole drawing every frame (99% of the main
  thread in a window). Animations now step on about ten times a second by default
  ("light", 5% idle); the new toolbar button switches light / smooth / off and the browser
  remembers it; [build] animation sets the default for built pages; view.animation().
  Pan and zoom gestures pause them. Status LEDs pause off-screen and with the rest now.
- Pan 28 -> 10 ms, zoom 69 -> 11 ms per step: gestures move the drawing as one compositor
  layer (transform) and set the viewBox once they end; --iso-k is set on the tags group
  only and only when it changes; the stage box is read once per frame.
- Hover 22 -> 15 ms per move: the tooltip is placed once per frame on its own layer and
  measured only when its text changes; the Studio's x/y readout updates ten times a second
  in a contained box; hover work in the Studio runs once per frame.
- Long object lists skip layout of rows out of sight (content-visibility).
- WASM built with opt-level 3: engine render 11.7 -> 9.4 ms for 10% more download.

Checks: Studio 102/102 (animation modes, pan as a layer, contained readout), web 87/87.

viewer/page.html

@@ -48,6 +48,7 @@ document.documentElement.setAttribute("data-theme",t);})();
   function esc(s) { return String(s).replace(/&/g, "&amp;").replace(/</g, "&lt;").replace(/>/g, "&gt;"); }
   var view = IsoKit.mount(document.getElementById("view"), IsoKit.staticSource(data), {
     fullscreen: document.querySelector(".card"),
+    animation: data.animation || undefined,
     onPick: function (i) {
       if (!i) { picked.textContent = "Tap or click anything for its details."; return; }
       picked.innerHTML = "<b>" + esc(i.name) + "</b>" + (i.detail ? "<br>" + esc(i.detail) : "") +