Support isolated live window ScreenCast streams

Render selected toplevels through a private portal-only capture protocol, preserve logical snapshot geometry, and renegotiate PipeWire video on window resizing. Bound outstanding snapshots and retry temporary contention; stop on source closure and session locking.

Validation: portal unit tests (31), capture budget unit, private-bus portal contracts, and real nested/PipeWire isolation, workspace, resize, close and paused-lock tests pass. Independent review cleared. Monitor restore integration timed out on both this build and the unchanged previous compositor under the current nested host.

compositor/src/ipc.rs

@@ -868,6 +868,8 @@ impl Ferese {
                     "workspace": self.workspaces.workspace_for_window(*id).map(|workspace| workspace.0),
                     "x": rect.as_ref().map(|rect| rect.loc.x),
                     "y": rect.as_ref().map(|rect| rect.loc.y),
+                    "capture_width": window.geometry().size.w,
+                    "capture_height": window.geometry().size.h,
                     "width": rect.as_ref().map(|rect| rect.size.w),
                     "height": rect.as_ref().map(|rect| rect.size.h),
                 }))