Speed up per-frame layout, hit testing, and stacking paths

Split ScrollingLayout validation so the per-frame geometry solve only
checks scalars and keeps the full tree validation behind debug_assert,
removing a HashSet build from every frame. Replace the per-region Vec
in decode_regions with a stack array via as_chunks, and skip the window
hit test in PointerMotion when Overview is not presenting, where
visual_scale_for_window returns 1.0 regardless.

Guard sync_window_stacking behind a check that the space already matches
the target order with uniform z_index, avoiding an O(n^2) raise_element
loop on every redraw. Narrow window_under_visual to the space iterator
outside Overview, keeping the collected and sorted candidate list for
the Overview branch where front-to-back order is load-bearing.

Raise rust-version to 1.98 and clear the resulting Clippy findings
across the workspace: let-chain collapses, clamp, and while-let loops.

compositor/src/handlers/compositor.rs

@@ -110,19 +110,6 @@ impl CompositorHandler for Ferese {
     }
 }
 
-#[cfg(test)]
-mod tests {
-    use super::*;
-    use smithay::wayland::shell::wlr_layer::Layer;
-    #[test]
-    fn panel_commits_do_not_invalidate_their_own_backdrop() {
-        assert!(!layer_affects_backdrop(Layer::Top));
-        assert!(!layer_affects_backdrop(Layer::Overlay));
-        assert!(layer_affects_backdrop(Layer::Bottom));
-        assert!(layer_affects_backdrop(Layer::Background));
-    }
-}
-
 impl BufferHandler for Ferese {
     fn buffer_destroyed(&mut self, _buffer: &wl_buffer::WlBuffer) {}
 }
@@ -148,3 +135,16 @@ impl ShmHandler for Ferese {
         &self.shm_state
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use smithay::wayland::shell::wlr_layer::Layer;
+    #[test]
+    fn panel_commits_do_not_invalidate_their_own_backdrop() {
+        assert!(!layer_affects_backdrop(Layer::Top));
+        assert!(!layer_affects_backdrop(Layer::Overlay));
+        assert!(layer_affects_backdrop(Layer::Bottom));
+        assert!(layer_affects_backdrop(Layer::Background));
+    }
+}