feat(iced): implement selection menu operations

- Add Inverse selection (Shift+Ctrl+I) with engine.invert_selection()
- Add Shrink, Feather, Border, Smooth menu items with parameter dialogs
- Implement selection_border() and selection_smooth() in engine API
- Implement border_mask() and smooth_mask() in hcie-selection crate
- Update SelectionOpApply handler for all selection operations

hcie-engine-api/src/lib.rs

@@ -412,6 +412,36 @@ impl Engine {
         }
     }
 
+    /// Create a border from the current selection.
+    ///
+    /// **Purpose:** Extracts only the edge pixels of the selection, keeping
+    /// pixels that are selected but have at least one unselected neighbor.
+    ///
+    /// **Arguments:**
+    /// - `px`: Border thickness in pixels.
+    pub fn selection_border(&mut self, px: u32) {
+        if let Some(ref mut mask) = self.document.selection_mask {
+            let w = self.document.canvas_width;
+            let h = self.document.canvas_height;
+            hcie_selection::border_mask(mask, w, h, px);
+        }
+    }
+
+    /// Smooth the current selection edges.
+    ///
+    /// **Purpose:** Softens jagged selection edges by applying a box blur
+    /// within the specified radius.
+    ///
+    /// **Arguments:**
+    /// - `px`: Blur radius in pixels.
+    pub fn selection_smooth(&mut self, px: u32) {
+        if let Some(ref mut mask) = self.document.selection_mask {
+            let w = self.document.canvas_width;
+            let h = self.document.canvas_height;
+            hcie_selection::smooth_mask(mask, w, h, px);
+        }
+    }
+
     pub fn selection_invert(&mut self) {
         if let Some(ref mut mask) = self.document.selection_mask {
             hcie_selection::invert_mask(mask);