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/main.rs

@@ -107,10 +107,10 @@ fn main() -> Result<(), Box<dyn Error>> {
     let reload_handle = event_loop.handle();
     let result = event_loop.run(None, &mut state, |state| {
         if let Some(monitor) = &mut monitor {
-            if let Some(result) = monitor.poll(std::time::Instant::now()) {
-                if let Err(error) = result.and_then(|source| state.reload_config_source(source)) {
-                    warn!(%error, "config reload rejected; retaining last working config");
-                }
+            if let Some(result) = monitor.poll(std::time::Instant::now())
+                && let Err(error) = result.and_then(|source| state.reload_config_source(source))
+            {
+                warn!(%error, "config reload rejected; retaining last working config");
             }
             // At most one timer exists. Edits arriving during debounce move the
             // deadline; the old timer wakes once and is replaced if necessary.