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/raster_test.rs

@@ -0,0 +1,44 @@
+//! Pure Cycle 3 raster behavior tests.
+
+use hcie_iced_gui::raster::apply_gradient;
+
+#[test]
+fn linear_gradient_interpolates_and_respects_mask() {
+    let mut pixels = vec![0; 3 * 4];
+    let mask = [255, 255, 0];
+    apply_gradient(
+        &mut pixels,
+        3,
+        1,
+        (0.5, 0.5),
+        (2.5, 0.5),
+        false,
+        [0, 0, 0, 255],
+        [200, 100, 50, 255],
+        1.0,
+        Some(&mask),
+    );
+    assert_eq!(&pixels[0..4], &[0, 0, 0, 255]);
+    assert_eq!(&pixels[4..8], &[100, 50, 25, 255]);
+    assert_eq!(&pixels[8..12], &[0, 0, 0, 0]);
+}
+
+#[test]
+fn radial_gradient_uses_distance_from_origin() {
+    let mut pixels = vec![0; 3 * 4];
+    apply_gradient(
+        &mut pixels,
+        3,
+        1,
+        (0.5, 0.5),
+        (2.5, 0.5),
+        true,
+        [0, 0, 0, 255],
+        [200, 0, 0, 255],
+        1.0,
+        None,
+    );
+    assert_eq!(pixels[0], 0);
+    assert_eq!(pixels[4], 100);
+    assert_eq!(pixels[8], 200);
+}