feat: implement layer mask editing support with scratch buffer and coordinate normalization

hcie-engine-api/tests/layer_mask_test.rs

@@ -0,0 +1,33 @@
+use hcie_engine_api::Engine;
+
+#[test]
+fn test_layer_mask_api_and_editing() {
+    let mut engine = Engine::new(100, 100);
+    let layer_id = engine.active_layer_id();
+
+    // Initially layer has no mask
+    assert!(!engine.has_layer_mask(layer_id));
+    assert!(engine.get_layer_mask_pixels(layer_id).is_none());
+
+    // Add layer mask
+    engine.add_layer_mask(layer_id);
+    assert!(engine.has_layer_mask(layer_id));
+    let mask = engine.get_layer_mask_pixels(layer_id).expect("Mask pixels must exist");
+    assert_eq!(mask.len(), 100 * 100);
+    assert_eq!(mask[0], 255);
+
+    // Toggle mask editing mode
+    assert!(!engine.is_editing_mask());
+    engine.set_editing_mask(true);
+    assert!(engine.is_editing_mask());
+
+    // Set custom mask pixels
+    let custom_mask = vec![128u8; 100 * 100];
+    engine.set_layer_mask_pixels(layer_id, custom_mask.clone(), Some([0, 0, 100, 100]));
+    assert_eq!(engine.get_layer_mask_pixels(layer_id), Some(custom_mask.as_slice()));
+
+    // Remove layer mask
+    engine.remove_layer_mask(layer_id);
+    assert!(!engine.has_layer_mask(layer_id));
+    assert!(engine.get_layer_mask_pixels(layer_id).is_none());
+}