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-shell/src/notifications.rs

@@ -129,7 +129,9 @@ impl Server {
             title: limited(summary, 160),
             body: limited(body, 500),
             actions: actions
-                .chunks_exact(2)
+                .as_chunks::<2>()
+                .0
+                .iter()
                 .take(8)
                 .map(|pair| (limited(&pair[0], 128), limited(&pair[1], 40)))
                 .collect(),