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/recording.rs

@@ -80,18 +80,20 @@ impl Recorder {
                 let (send, receive) = mpsc::sync_channel(8);
                 self.updates = Some(receive);
                 self.state = State::Selecting;
+
                 std::thread::spawn(move || {
                     let mut output = BufReader::new(output);
+
                     loop {
                         let mut line = String::new();
                         match output.by_ref().take(65537).read_line(&mut line) {
                             Ok(0) | Err(_) => break,
                             Ok(_) if line.len() > 65536 => break,
                             Ok(_) => {
-                                if let Ok(update) = serde_json::from_str::<Update>(&line) {
-                                    if send.send(Event::Update(update)).is_err() {
-                                        break;
-                                    }
+                                if let Ok(update) = serde_json::from_str::<Update>(&line)
+                                    && send.send(Event::Update(update)).is_err()
+                                {
+                                    break;
                                 }
                             }
                         }