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-engine-api/tests/bitmap_brush_stamp.rs

@@ -1,11 +1,13 @@
-use hcie_engine_api::{Engine, BrushTip, BrushStyle};
+use hcie_engine_api::{BrushStyle, BrushTip, Engine};
 
 #[test]
 fn bitmap_brush_stamp_shape() {
     let mut engine = Engine::new(64, 64);
     // 4x4 solid white square bitmap stamp
     let mut pixels = vec![0u8; 16];
-    for i in 0..16 { pixels[i] = 255; }
+    for i in 0..16 {
+        pixels[i] = 255;
+    }
     let brush = BrushTip {
         style: BrushStyle::Bitmap,
         size: 8.0,
@@ -25,25 +27,46 @@ fn bitmap_brush_stamp_shape() {
 
     // Check that the painted area is roughly 4x4, not a circular blob.
     // Count alpha > 0 pixels and bounding box.
-    let mut min_x = 63; let mut max_x = 0; let mut min_y = 63; let mut max_y = 0;
+    let mut min_x = 63;
+    let mut max_x = 0;
+    let mut min_y = 63;
+    let mut max_y = 0;
     let mut count = 0;
     for y in 0..64 {
         for x in 0..64 {
-            let a = output[(y*64+x)*4+3];
+            let a = output[(y * 64 + x) * 4 + 3];
             if a > 0 {
                 count += 1;
-                if x < min_x { min_x = x; }
-                if x > max_x { max_x = x; }
-                if y < min_y { min_y = y; }
-                if y > max_y { max_y = y; }
+                if x < min_x {
+                    min_x = x;
+                }
+                if x > max_x {
+                    max_x = x;
+                }
+                if y < min_y {
+                    min_y = y;
+                }
+                if y > max_y {
+                    max_y = y;
+                }
             }
         }
     }
     let w = (max_x - min_x + 1) as i32;
     let h = (max_y - min_y + 1) as i32;
     // Should fit inside ~8x8 because size=8 and stamp 4x4 scaled up
-    assert!(w <= 8 && h <= 8, "bitmap dab bounding box {}x{} too large for 4x4 stamp", w, h);
+    assert!(
+        w <= 8 && h <= 8,
+        "bitmap dab bounding box {}x{} too large for 4x4 stamp",
+        w,
+        h
+    );
     assert!(count >= 8, "bitmap dab should paint some pixels");
     // Square-ish: aspect ratio not too far from 1 (was very low for circle vs square?)
-    assert!(w.abs_diff(h) <= 2, "bitmap dab bounding box should be roughly square, got {}x{}", w, h);
+    assert!(
+        w.abs_diff(h) <= 2,
+        "bitmap dab bounding box should be roughly square, got {}x{}",
+        w,
+        h
+    );
 }