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-brush-engine/tests/stamp.rs

@@ -1,4 +1,4 @@
-use hcie_brush_engine::{BrushTip, BrushStyle};
+use hcie_brush_engine::{BrushStyle, BrushTip};
 
 fn make_round_tip(size: f32, hardness: f32) -> BrushTip {
     BrushTip {
@@ -14,7 +14,11 @@ fn test_generate_brush_stamp_size() {
     let tip = make_round_tip(10.0, 0.5);
     let stamp = hcie_brush_engine::generate_brush_stamp(&tip);
     let expected_d = 20usize;
-    assert_eq!(stamp.len(), expected_d * expected_d, "stamp should be square");
+    assert_eq!(
+        stamp.len(),
+        expected_d * expected_d,
+        "stamp should be square"
+    );
 }
 
 #[test]
@@ -30,7 +34,11 @@ fn test_generate_brush_stamp_center_nonzero() {
 fn test_generate_brush_stamp_hardness_gradient() {
     let soft = hcie_brush_engine::generate_brush_stamp(&make_round_tip(10.0, 0.0));
     let hard = hcie_brush_engine::generate_brush_stamp(&make_round_tip(10.0, 1.0));
-    assert_eq!(soft.len(), hard.len(), "same size stamp should have same length");
+    assert_eq!(
+        soft.len(),
+        hard.len(),
+        "same size stamp should have same length"
+    );
 }
 
 #[test]
@@ -38,7 +46,11 @@ fn test_brush_spacing_pixels() {
     let spacing = hcie_brush_engine::brush_spacing_pixels(20.0, 0.5);
     assert_eq!(spacing, 10.0, "spacing should be size * ratio");
     let min_spacing = hcie_brush_engine::brush_spacing_pixels(20.0, 0.0);
-    assert!((min_spacing - 0.2).abs() < 0.001, "min spacing should be ~0.2, got {}", min_spacing);
+    assert!(
+        (min_spacing - 0.2).abs() < 0.001,
+        "min spacing should be ~0.2, got {}",
+        min_spacing
+    );
 }
 
 #[test]