Implement stable serialization and restoration for dock layouts, including auto-hidden and floating panels

- Add `persistence.rs` for stable serialization of dock layouts without runtime identifiers.
- Introduce `preview.rs` for geometry handling of dock drop previews.
- Create `sizing.rs` to manage content-aware sizing policies and constraint solving for dock panels.
- Implement `welcome.rs` to provide a welcome surface when no documents are open, featuring New and Open actions.

tools/screenshot-diff/src/main.rs

@@ -8,12 +8,23 @@ fn main() {
     }
     let target_path = PathBuf::from(&args[1]);
     let actual_path = PathBuf::from(&args[2]);
-    let save_diff = args.iter().position(|a| a == "--save-diff").and_then(|i| args.get(i + 1)).map(PathBuf::from);
+    let save_diff = args
+        .iter()
+        .position(|a| a == "--save-diff")
+        .and_then(|i| args.get(i + 1))
+        .map(PathBuf::from);
 
-    let target = image::open(&target_path).expect("Failed to open target").to_rgba8();
-    let actual = image::open(&actual_path).expect("Failed to open actual").to_rgba8();
+    let target = image::open(&target_path)
+        .expect("Failed to open target")
+        .to_rgba8();
+    let actual = image::open(&actual_path)
+        .expect("Failed to open actual")
+        .to_rgba8();
 
-    let (w, h) = (target.width().min(actual.width()), target.height().min(actual.height()));
+    let (w, h) = (
+        target.width().min(actual.width()),
+        target.height().min(actual.height()),
+    );
     if w == 0 || h == 0 {
         eprintln!("One of the images has zero size");
         std::process::exit(1);