Clear WS_CLIPCHILDREN on the client's window

The client creates its 3D view's window with it, so not adding it wasn't
enough: covered entirely by the renderer's window, the client's window got no
WM_PAINT and the client still drew about once a second.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

docs/architecture.md

@@ -30,7 +30,8 @@ creates (each 3D view, and again on room changes), the adapter:
 The renderer process draws into a child window it creates inside the client's window: window handles are valid in
 every process, and Windows allows child windows across processes (Chrome's GPU process works this way). The child
 is disabled so mouse input goes to the client's window under it. Its swap chain is a flip model one, which the
-compositor shows above the client's own drawing. The client's window must not clip its children (`WS_CLIPCHILDREN`):
+compositor shows above the client's own drawing. The client's window must not clip its children (`WS_CLIPCHILDREN`,
+which the client creates it with, so the adapter clears it):
 the client draws a frame when its window gets `WM_PAINT` (`SceneWindow.dll` invalidates it, and draws on the paint),
 and a window entirely covered by clipped children never gets one, so the client fell to about one frame a second.
 The process exits when the engine is destroyed or the client exits.