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.

crates/isokit-project/src/assets.rs

@@ -78,6 +78,7 @@ label = "Animation"
 [build]
 out = "dist/index.html"
 budget = 4000
+# animation = "light"   # built pages: "light" (about 10 steps a second), "smooth" or "off"
 "#;
 
 pub const NEW_DOC: &str = r#"{

crates/isokit-project/src/lib.rs

@@ -83,6 +83,10 @@ pub struct BuildOpts {
     /// Node budget per view for `isokit check`.
     #[serde(default = "budget")]
     pub budget: usize,
+    /// How built pages animate until the viewer picks otherwise: "light"
+    /// (stepped about ten times a second, the default), "smooth" or "off".
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub animation: Option<String>,
 }
 
 fn budget() -> usize {
@@ -94,6 +98,7 @@ impl Default for BuildOpts {
         BuildOpts {
             out: None,
             budget: budget(),
+            animation: None,
         }
     }
 }

crates/isokit-project/src/page.rs

@@ -32,6 +32,7 @@ pub fn static_page(p: &Project) -> Result<String, ScriptError> {
         },
         "views": views,
         "cfg": p.cfg(None),
+        "animation": p.manifest.build.animation,
     });
     let css = format!(
         "{}\n{}",