fix(ui): the overlay layers follow the interface root's size

[davidc-smith]

Problem

After entering the world, plugin canvases are clipped to the top-left 800x600 of the screen.

RetailUiRuntime.RestoreLayout() runs when the character enters the world. That is when the window switches from the pre-game 800x600 to the game resolution. It sizes the interface root and records the new size in _lastScreenSize, but never tells the overlay host. The next Draw sees no change, so its guarded SetViewport(screenSize) never runs, and every overlay layer keeps its 800x600 rectangle. Layers clip their children, so a canvas outside that rectangle is cut off.

It looks intermittent because the world label and projectile overlays call SetViewport(camera.Viewport) whenever they have something to draw. While labels are on screen, they push the right size in and hide the bug.

Fix

• UiOverlayHost.SetViewport is replaced by FollowRoot(), which takes its size from the interface root. RetailUiRuntime.Draw calls it every frame, so a resize made anywhere reaches the layers.
• The world overlays no longer write the host's shared rectangle. They still size their own elements from the camera.

Tests

• UiOverlayHostTests: layers follow a root resized behind the host's back, and a world overlay ticking with a different camera viewport leaves the layers at screen size.
• The existing canvas anchor test now resizes through the root.
• I checked it live on macOS with a plugin canvas. Before the fix, the overlay layers stayed at 800x600 after the login resize. After it, they followed the new window size.

https://claude.ai/code/session_01SY57mHJLZ6djpWZ5nP37Tw