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/private_client.rs

@@ -16,6 +16,7 @@ pub(crate) struct ClientCapabilities(u8);
 impl ClientCapabilities {
     pub(crate) const EFFECTS: Self = Self(1 << 0);
     pub(crate) const SHELL_CONTROL: Self = Self(1 << 1);
+    pub(crate) const WINDOW_CAPTURE: Self = Self(1 << 2);
 
     pub(crate) fn contains(self, capability: Self) -> bool {
         self.0 & capability.0 == capability.0