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_servo_host/src/lib.rs

@@ -19,12 +19,12 @@ mod runtime_webview;
 pub use error::ServoHostError;
 #[cfg(feature = "hardware-render")]
 pub use hardware_rendering_context::HardwareOffscreenContext;
-pub use iosurface_handle::{IOSurfaceHandle, IOSurfaceIdentity};
 pub use host::{
-    HidpiScaleRequest, KeyboardTextRequest, MouseClickRequest, MouseDragRequest,
-    MouseHoverRequest, NavigationRequest, PageZoomRequest, PermissionDecision, PermissionRequest,
-    RenderedFrame, RenderedFrameSummary, ResizeRequest, ScreenshotRequest, ScrollRequest,
-    ServoHost, TouchTapRequest, WebViewSnapshot, WebViewState,
+    HidpiScaleRequest, KeyboardTextRequest, MouseClickRequest, MouseDragRequest, MouseHoverRequest,
+    NavigationRequest, PageZoomRequest, PermissionDecision, PermissionRequest, RenderedFrame,
+    RenderedFrameSummary, ResizeRequest, ScreenshotRequest, ScrollRequest, ServoHost,
+    TouchTapRequest, WebViewSnapshot, WebViewState,
 };
+pub use iosurface_handle::{IOSurfaceHandle, IOSurfaceIdentity};
 #[cfg(feature = "servo-engine")]
 pub use runtime::{RenderingContextKind, ServoSurfaceSize, SoftwareServoHost};