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/backends/direct.rs

@@ -1101,7 +1101,7 @@ fn select_outputs(drm: &DrmDevice, profiles: &[OutputProfile]) -> io::Result<Out
             enabled: settings.enabled,
             profile: active_profile.map(|profile| profile.name.clone()),
             physical_size: connector.size(),
-            current_mode: selected_mode.map(|mode| connected_mode_info(mode)),
+            current_mode: selected_mode.map(connected_mode_info),
             available_modes: connector
                 .modes()
                 .iter()