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/handlers/mod.rs

@@ -5,6 +5,7 @@ mod layer_shell;
 pub(crate) mod screencopy;
 pub(crate) mod screenshot;
 pub(crate) mod screenshot_worker;
+pub(crate) mod window_capture;
 mod xdg_shell;
 
 use smithay::{