feat(gui): implement selection mask state and vector editing functionality

- Add `selection/state.rs` to manage selection masks, including combining masks and calculating bounds.
- Introduce `vector_edit.rs` for vector shape manipulation, handling drag sessions, and hit testing for handles.
- Create `feature_scorecard.rs` to ensure stable feature identifiers and regression gates for GUI components.
- Implement raster behavior tests in `raster_test.rs` to validate gradient application against masks.

hcie-iced-app/crates/hcie-iced-gui/tests/selection_test.rs

@@ -1,6 +1,8 @@
 //! Tests for selection types: SelectionTransform, TransformHandle, CropState.
 
-use hcie_iced_gui::selection::{CropState, SelectionTransform, TransformHandle};
+use hcie_iced_gui::selection::clipboard::centered_transform;
+use hcie_iced_gui::selection::state::{combine_masks, mask_bounds, selected_spans};
+use hcie_iced_gui::selection::{CropState, SelectionMode, SelectionTransform, TransformHandle};
 
 #[test]
 fn test_selection_transform_is_empty() {
@@ -70,3 +72,53 @@ fn test_transform_handle_cursor_hints() {
     assert_eq!(TransformHandle::TopLeft.cursor_hint(), "nwse-resize");
     assert_eq!(TransformHandle::Rotate.cursor_hint(), "crosshair");
 }
+
+#[test]
+fn selection_modes_combine_alpha_without_losing_soft_edges() {
+    let old = [0, 64, 255, 255];
+    let new = [255, 128, 64, 0];
+    assert_eq!(combine_masks(Some(&old), &new, SelectionMode::Replace), new);
+    assert_eq!(
+        combine_masks(Some(&old), &new, SelectionMode::Add),
+        [255, 128, 255, 255]
+    );
+    assert_eq!(
+        combine_masks(Some(&old), &new, SelectionMode::Subtract),
+        [0, 0, 191, 255]
+    );
+    assert_eq!(
+        combine_masks(Some(&old), &new, SelectionMode::Intersect),
+        [0, 64, 64, 0]
+    );
+}
+
+#[test]
+fn irregular_mask_bounds_and_fill_spans_are_exact() {
+    let mask = [0, 255, 0, 0, 255, 255, 0, 0, 0, 255, 0, 0];
+    assert_eq!(mask_bounds(&mask, 4, 3), Some((0, 0, 2, 3)));
+    assert_eq!(
+        selected_spans(&mask, 4, 3),
+        vec![(1, 0, 2), (0, 1, 2), (1, 2, 2)]
+    );
+}
+
+#[test]
+fn crop_clamps_drag_and_cancel_restores_idle_state() {
+    let mut crop = CropState::default();
+    crop.start_drag_clamped(-20.0, 5.0, 100, 80);
+    crop.update_drag(140.0, 120.0);
+    assert_eq!(crop.rect, Some((0.0, 5.0, 100.0, 75.0)));
+    crop.cancel();
+    assert_eq!(crop.canvas_size, None);
+    assert!(!crop.active);
+}
+
+#[test]
+fn clipboard_transform_is_centered_and_transform_geometry_is_safe() {
+    let mut transform = centered_transform(vec![255; 8 * 4 * 4], 8, 4, 100, 50).unwrap();
+    assert_eq!(transform.pos, iced::Vector::new(46.0, 23.0));
+    transform.size = iced::Vector::new(-20.0, f32::NAN);
+    transform.sanitize_geometry(true);
+    assert!(transform.size.x > 0.0 && transform.size.y > 0.0);
+    assert!((transform.size.x / transform.size.y - 2.0).abs() < 0.001);
+}