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.

studio/studio.js

@@ -251,7 +251,8 @@
   }
 
   // ------------------------------------------------------------ view
-  var drag = null, tool = null;
+  var drag = null, tool = null, hov = null, hovRaf = 0, hovText = "", hovLate = 0;
+  function showCursor() { hovLate = 0; var el = $("cursor"); if (el.textContent !== hovText) el.textContent = hovText; }
   function moveEls(id) { return S.view.svg.querySelectorAll('[data-obj="' + id.replace(/"/g, "") + '"]'); }
   function mountView() {
     var src = B ? B.source(toViewerMeta(S.meta)) : IsoKit.httpSource("", toViewerMeta(S.meta));
@@ -264,9 +265,15 @@
         var w = $("warnbtn"); w.hidden = !S.warnings.length; w.textContent = S.warnings.length + " warning" + (S.warnings.length === 1 ? "" : "s");
         var ids = selIds(); if (ids.length) S.view.selectObj(ids.length > 1 ? ids : ids[0]);
       },
+      // once per frame at most: pointers send moves faster than that; the x/y readout about ten
+      // times a second (each new text costs a layout and a paint of the page)
       onHover: function (w, e) {
-        if (w) $("cursor").textContent = "x " + w[0].toFixed(1) + "  y " + w[1].toFixed(1);
-        if (S.placing && !tool && e) hoverPlacing(e);
+        hov = { w: w, e: e };
+        if (!hovRaf) hovRaf = requestAnimationFrame(function () {
+          var h = hov; hovRaf = 0;
+          if (h.w) { hovText = "x " + h.w[0].toFixed(1) + "  y " + h.w[1].toFixed(1); if (!hovLate) hovLate = setTimeout(showCursor, 100); }
+          if (S.placing && !tool && h.e) hoverPlacing(h.e);
+        });
       },
       onPick: function (i, e) { if (i && i.obj) clickObj(i.obj, e); else select(null); },
       onKey: keyHandler,