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-settings/src/visuals.rs

@@ -451,7 +451,12 @@ pub fn preview(snapshot: &Snapshot) -> Element<'static, Message> {
     let bar_height = snapshot.number("theme.geometry.top_bar_height", 30.) * 0.65;
     let bar_radius = snapshot.number("theme.geometry.shell_radius", 14.) * 0.65;
     let opacity = if snapshot.string("theme.material.style", "solid") == "translucent" {
-        snapshot.number("theme.material.opacity", ferese_config::DEFAULT_MATERIAL_OPACITY).clamp(0., 1.)
+        snapshot
+            .number(
+                "theme.material.opacity",
+                ferese_config::DEFAULT_MATERIAL_OPACITY,
+            )
+            .clamp(0., 1.)
     } else {
         1.
     };
@@ -556,11 +561,12 @@ mod tests {
     fn default_uses_logo_blue_and_native_controls_follow_lightness() {
         let logo = include_str!("../../../packaging/icons/ferese.svg").to_ascii_uppercase();
         assert!(logo.contains(PRESETS[0].accent));
-        for index in 0..PRESETS.len() {
+
+        for (index, item) in PRESETS.iter().enumerate() {
             let snapshot = configured_preset(index);
             assert_eq!(
                 native_theme(Some(&snapshot)).cosmic().is_dark,
-                PRESETS[index].name != "Ayu Light"
+                item.name != "Ayu Light"
             );
         }
     }