Move Servo IPC off UI thread

Root cause of the post-tab lag: the GPUI 16 ms timer was calling
`WebSurfaceRuntime::ensure_tab` and `tick` on the UI thread, and each
call did a synchronous `serde_json` write plus `read_line` against the
Servo sidecar over stdin/stdout. With even one visible tab, every
frame stalled on cross-process IPC.

Introduce `web_surface_worker.rs` — a per-profile worker thread that
owns the `ServoLiveClient`, drains a coalescing request queue
(latest Ensure/Poll per tab wins, no unbounded growth), and ships
results back through a `std::sync::mpsc` channel. `WebSurfaceRuntime`
now submits work non-blockingly and drains responses in `tick`; the
UI thread never blocks on the sidecar.

Adjacent in-flight cleanup riding along: hardware IOSurface
rendering-context completion (sidecar `live_protocol`,
`hardware_rendering_context`, GPUI BGRA surface shader), CSS viewport
size + device pixel ratio plumbing into `ServoLiveFrame`, and the
Send opt-ins for `CVPixelBuffer`-bearing types so frames can cross
the thread boundary.

crates/ely_app/src/shell/mod.rs

@@ -32,6 +32,7 @@ mod web_surface_permissions;
 mod web_surface_runtime;
 mod web_surface_state;
 mod web_surface_view;
+mod web_surface_worker;
 
 #[cfg(test)]
 mod gpui_harness_tests;
@@ -116,9 +117,8 @@ impl ElyShell {
         window: &mut Window,
         cx: &mut Context<Self>,
     ) -> Self {
-        let command_input = cx.new(|cx| {
-            InputState::new(window, cx).placeholder("Search ELY or type a command…")
-        });
+        let command_input =
+            cx.new(|cx| InputState::new(window, cx).placeholder("Search ELY or type a command…"));
         let plugin_search_input =
             cx.new(|cx| InputState::new(window, cx).placeholder("Search plugins…"));
         let translucency_slider = cx.new(|_cx| {
@@ -129,14 +129,16 @@ impl ElyShell {
                 .default_value(f32::from(DEFAULT_TRANSLUCENCY_PCT))
         });
 
-        let translucency_subscription =
-            cx.subscribe(&translucency_slider, |shell: &mut Self, _state, event: &SliderEvent, cx| {
+        let translucency_subscription = cx.subscribe(
+            &translucency_slider,
+            |shell: &mut Self, _state, event: &SliderEvent, cx| {
                 let SliderEvent::Change(SliderValue::Single(value)) = event else {
                     return;
                 };
                 let pct = value.clamp(0.0, 100.0).round() as u8;
                 shell.set_translucency_pct(pct, cx);
-            });
+            },
+        );
 
         let command_subscription = cx.subscribe_in(
             &command_input,