[client] cursor: restore Wayland edge synchronization

Keep guest-to-host pointer synchronization active while an attempted
surface exit is blocked by the edge of a Wayland output. Continue
forwarding relative motion and wait for wl_pointer.leave before marking
the cursor outside the window.

Distinguish whole-surface releases from exits into letterbox space so
the expected asynchronous release callback cannot disable the feedback
loop before the pointer is confined again.

Treat Wayland pointer capability loss as a real leave so an unrelated
inactive callback cannot be mistaken for the intentional release.

client/src/main.h

@@ -301,6 +301,9 @@ struct CursorState
   /* true if a pointer exit is waiting for confinement to release */
   bool exit;
 
+  /* true if a Wayland surface exit is waiting for a real leave */
+  bool surfaceExit;
+
   /* the local pointer exit target */
   struct DoublePoint exitPos;