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/overview.rs

@@ -92,6 +92,14 @@ pub(crate) fn init_font_loader(
     Ok(())
 }
 
+type OverviewStateLabel = HashMap<
+    (String, u64, [u32; 4]),
+    (
+        MemoryRenderBuffer,
+        smithay::utils::Size<i32, smithay::utils::Buffer>,
+    ),
+>;
+
 #[derive(Debug)]
 pub(crate) struct OverviewState {
     active: bool,
@@ -103,13 +111,7 @@ pub(crate) struct OverviewState {
     font_family: String,
     font_requests: Option<FontRequests>,
     font: Option<FontArc>,
-    labels: HashMap<
-        (String, u64, [u32; 4]),
-        (
-            MemoryRenderBuffer,
-            smithay::utils::Size<i32, smithay::utils::Buffer>,
-        ),
-    >,
+    labels: OverviewStateLabel,
 }
 
 impl Default for OverviewState {
@@ -480,6 +482,7 @@ impl Ferese {
         let selected = self
             .window_under_visual(point)
             .and_then(|w| self.window_ids.get(&w).copied());
+
         if selected.is_some() && self.overview.selected != selected {
             self.overview.selected = selected;
             crate::backends::direct::render_all(self);
@@ -655,8 +658,7 @@ impl Ferese {
         let card_width =
             ((strip.width - 24.0 - WORKSPACE_CARD_GAP * capacity.saturating_sub(1) as f64)
                 / capacity as f64)
-                .min(160.0)
-                .max(1.0);
+                .clamp(1.0, 160.0);
         let visible_count = workspaces.len().saturating_sub(offset).min(capacity);
         let start_x = centered_row_start(strip, visible_count, card_width);
         workspaces