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.

shell/ferese-lock/src/appearance.rs

@@ -109,12 +109,14 @@ impl Appearance {
         } else {
             ThemeBuilder::dark()
         };
-        let mut corners = cosmic::cosmic_theme::CornerRadii::default();
-        corners.radius_xs = [self.radius.min(4.); 4];
-        corners.radius_s = [self.radius.min(8.); 4];
-        corners.radius_m = [self.radius; 4];
-        corners.radius_l = [self.radius; 4];
-        corners.radius_xl = [self.radius; 4];
+        let corners = cosmic::cosmic_theme::CornerRadii {
+            radius_0: Default::default(),
+            radius_xs: [self.radius.min(4.); 4],
+            radius_s: [self.radius.min(8.); 4],
+            radius_m: [self.radius; 4],
+            radius_l: [self.radius; 4],
+            radius_xl: [self.radius; 4],
+        };
         let mut native = builder
             .corner_radii(corners)
             .bg_color(color(self.panel))