Plug three holes in Servo input forwarding

1. Scroll no longer wipes keyboard focus. Servo holds DOM focus across
   wheel events; the shell was clearing keyboard_focus and typed_texts
   on every scroll, so a focused input went deaf the moment the user
   scrolled. Scroll still drops the buffered click point because that
   coordinate is captured against the pre-scroll viewport.

2. Mouse-down hands focus to the shell's root focus handle (in
   addition to mouse-up's existing click forwarding). The user can now
   start typing the moment they press the page, instead of having to
   first complete a click round-trip to escape the omnibar's focus.

3. Sidecar hover() honors the requesting webview_id instead of
   defaulting to the first webview in the map, so multi-tab sidecars
   no longer pipe every hover into tab #1.

crates/ely_servo_host/src/lib.rs

@@ -15,9 +15,10 @@ mod runtime_webview;
 
 pub use error::ServoHostError;
 pub use host::{
-    KeyboardTextRequest, MouseClickRequest, MouseDragRequest, NavigationRequest, PageZoomRequest,
-    PermissionDecision, PermissionRequest, RenderedFrame, RenderedFrameSummary, ResizeRequest,
-    ScreenshotRequest, ScrollRequest, ServoHost, TouchTapRequest, WebViewSnapshot, WebViewState,
+    KeyboardTextRequest, MouseClickRequest, MouseDragRequest, MouseHoverRequest,
+    NavigationRequest, PageZoomRequest, PermissionDecision, PermissionRequest, RenderedFrame,
+    RenderedFrameSummary, ResizeRequest, ScreenshotRequest, ScrollRequest, ServoHost,
+    TouchTapRequest, WebViewSnapshot, WebViewState,
 };
 #[cfg(feature = "servo-engine")]
 pub use runtime::{ServoSurfaceSize, SoftwareServoHost};