rml-compositing W3: click/tap-to-focus (kernel on_pressed + window-field)

Completes the window-manager input story.

kernel (additive): SurfaceElement::on_pressed(std::function<void()>) — the
substrate invokes it (error-isolated to the owner) when a pointer button PRESS
or touch DOWN is routed to that surface element (root or any subsurface/popup
child fires the root element's handler), in addition to the existing client
input-back forwarding. Not fired on motion/release/miss. It is the
click/tap-to-focus SIGNAL; focus policy stays the wm's.

ext-window-field: on map, sets the element's on_pressed -> that toplevel's
Toplevel::focus() (keyboard focus + on_toplevel_focused -> RCSS raise/highlight),
guarded on the window still being tracked. So a click/tap on a background window
now focuses it (the gap noted in Wave 3).

kernel + ext-window-field + ext-xdg-shell suites green; build-asan green, no new
unbox::-framed leak/UB (the stored handler dies with the element).

1 files changed, 3 insertions, 2 deletions

diff --git a/packages/kernel/src/server.cpp b/packages/kernel/src/server.cpp
index f88fa8f..d4f47c8 100644
--- a/packages/kernel/src/server.cpp
+++ b/packages/kernel/src/server.cpp
@@ -107,7 +107,8 @@ auto Server::ui_create_surface_element_for_test() -> bool {
         return false;
     }
     impl_->test_surface_element =
-        impl_->substrate->create_surface_element(impl_->test_last_client_surface);
+        impl_->substrate->create_surface_element(kernel_extension_id,
+                                                 impl_->test_last_client_surface);
     return impl_->test_surface_element != nullptr;
 }
 
@@ -261,7 +262,7 @@ auto PerExtensionUi::create_surface_element(wlr_surface* client)
     if (server_->substrate == nullptr) {
         return nullptr;
     }
-    return server_->substrate->create_surface_element(client);
+    return server_->substrate->create_surface_element(id_, client);
 }
 
 auto PerExtensionUi::available() const -> bool {