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/notification_ui.rs

@@ -520,7 +520,6 @@ impl FereseShell {
                     color: color_with_opacity(palette.text_muted, 0.18 * opacity),
                     width: if history { 0.0 } else { 1.0 },
                     radius: palette.material_radius.into(),
-                    ..Default::default()
                 },
                 ..Default::default()
             }));