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.

hcie-io/examples/inspect_diff.rs

@@ -1,7 +1,9 @@
 use image::GenericImageView;
 
 fn main() {
-    let ref_img = image::open("_images/_psd_stil_test/test_2/Emboss.png").unwrap().to_rgba8();
+    let ref_img = image::open("_images/_psd_stil_test/test_2/Emboss.png")
+        .unwrap()
+        .to_rgba8();
     let test_img = image::open("/tmp/test2_Emboss.png").unwrap().to_rgba8();
 
     let mut diff_sum = 0.0;
@@ -15,10 +17,10 @@ fn main() {
             let rp = ref_img.get_pixel(x, y);
             let tp = test_img.get_pixel(x, y);
 
-            let d = (rp[0] as i32 - tp[0] as i32).abs() +
-                    (rp[1] as i32 - tp[1] as i32).abs() +
-                    (rp[2] as i32 - tp[2] as i32).abs() +
-                    (rp[3] as i32 - tp[3] as i32).abs();
+            let d = (rp[0] as i32 - tp[0] as i32).abs()
+                + (rp[1] as i32 - tp[1] as i32).abs()
+                + (rp[2] as i32 - tp[2] as i32).abs()
+                + (rp[3] as i32 - tp[3] as i32).abs();
 
             diff_sum += d as f64;
             max_diff = max_diff.max(d);