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.

tools/check_web.py

@@ -300,6 +300,14 @@ try:
         check(pg.evaluate("() => __studio.S.warnings.length") == 0 and pg.locator('.st-tile[data-type="@room"]').count() == 1,
               "it draws without warnings, the palette has the Room tool")
         shot(pg, "web-house.jpg")
+        # low-power default: the pond's fish (and every other animation) step on about ten times a
+        # second instead of running as CSS animations that repaint the drawing every frame
+        an = pg.evaluate("""() => { const a = document.querySelector('.iso-svg').getAnimations({subtree: true});
+            return [__studio.view.animation(), a.length, a.filter(x => x.playState === 'running').length, a.length ? a[0].currentTime : 0]; }""")
+        pg.wait_for_timeout(500)
+        t1 = pg.evaluate("() => { const a = document.querySelector('.iso-svg').getAnimations({subtree: true}); return a.length ? a[0].currentTime : 0; }")
+        check(an[0] == "light" and an[1] >= 6 and an[2] == 0 and t1 > an[3] and pg.locator('.iso-bar [data-a="anim"]').is_visible(),
+              f"animations are light by default: {an[1]} stepped, none running, time moves on; the toolbar can switch")
 
         # ---------------------------------------------------------------- folder project (OPFS stands in for a picked folder)
         demo = {f.relative_to(ROOT / "examples" / "demo").as_posix(): f.read_text(encoding="utf-8")