[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/tests/CMakeLists.txt

@@ -66,6 +66,7 @@ target_link_libraries(mouse-tests
 )
 set(MOUSE_CASES
   inset-exit
+  surface-exit
   exit-delay
   exit-guest
   exit-cancel