fix: add selection_mask and selection_bounds fields to IcedDocument

These fields are required by the spec (section S2) but were missed during
initial implementation. They support selection mask alpha channel data
and selection bounding box coordinates.

hcie-iced-app/crates/hcie-iced-gui/src/app.rs

@@ -203,6 +203,10 @@ pub struct IcedDocument {
     pub transform_drag_handle: TransformHandle,
     /// Internal clipboard for copy/paste operations
     pub internal_clipboard: Option<SelectionTransform>,
+    /// Selection mask (alpha channel) for the current selection
+    pub selection_mask: Option<Vec<u8>>,
+    /// Selection bounds (x, y, width, height)
+    pub selection_bounds: Option<(u32, u32, u32, u32)>,
     /// Crop tool state
     pub crop_state: CropState,
 }
@@ -524,6 +528,8 @@ impl HcieIcedApp {
             selection_transform: None,
             transform_drag_handle: TransformHandle::None,
             internal_clipboard: None,
+            selection_mask: None,
+            selection_bounds: None,
             crop_state: CropState::default(),
         };
 
@@ -1804,6 +1810,8 @@ impl HcieIcedApp {
                     selection_transform: None,
                     transform_drag_handle: TransformHandle::None,
                     internal_clipboard: None,
+                    selection_mask: None,
+                    selection_bounds: None,
                     crop_state: CropState::default(),
                 });
                 self.active_doc = self.documents.len() - 1;
@@ -2223,6 +2231,8 @@ impl HcieIcedApp {
                     selection_transform: None,
                     transform_drag_handle: TransformHandle::None,
                     internal_clipboard: None,
+                    selection_mask: None,
+                    selection_bounds: None,
                     crop_state: CropState::default(),
                 });
             }