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/host.rs

@@ -242,6 +242,13 @@ pub struct MouseClickRequest {
     pub y: u32,
 }
 
+#[derive(Clone, Debug, Eq, PartialEq)]
+pub struct MouseHoverRequest {
+    pub webview_id: WebViewId,
+    pub x: u32,
+    pub y: u32,
+}
+
 #[derive(Clone, Debug, Eq, PartialEq)]
 pub struct MouseDragRequest {
     pub webview_id: WebViewId,