Add SelectionAttributes unit tests

Test core data type for selected/unselected state, default values,
ID constant, interface compliance, and instance independence.

8 tests covering all public API of SelectionAttributes.

Closes #6

src/test/java/ovh/gasser/newshapes/attributes/SelectionAttributesTest.java

@@ -0,0 +1,72 @@
+package ovh.gasser.newshapes.attributes;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class SelectionAttributesTest {
+
+    @Test
+    void testDefaultConstructorIsUnselected() {
+        SelectionAttributes attrs = new SelectionAttributes();
+        assertFalse(attrs.selected, "Default constructor should create unselected state");
+    }
+
+    @Test
+    void testConstructorWithTrue() {
+        SelectionAttributes attrs = new SelectionAttributes(true);
+        assertTrue(attrs.selected, "Constructor with true should create selected state");
+    }
+
+    @Test
+    void testConstructorWithFalse() {
+        SelectionAttributes attrs = new SelectionAttributes(false);
+        assertFalse(attrs.selected, "Constructor with false should create unselected state");
+    }
+
+    @Test
+    void testGetIDReturnsCorrectValue() {
+        SelectionAttributes attrs = new SelectionAttributes();
+        assertEquals(SelectionAttributes.ID, attrs.getID());
+    }
+
+    @Test
+    void testIDConstant() {
+        assertEquals("SELECTION_ATTRS", SelectionAttributes.ID);
+    }
+
+    @Test
+    void testImplementsAttributes() {
+        SelectionAttributes attrs = new SelectionAttributes();
+        assertInstanceOf(Attributes.class, attrs);
+    }
+
+    @Test
+    void testSelectedFieldCanBeToggled() {
+        SelectionAttributes attrs = new SelectionAttributes(false);
+        assertFalse(attrs.selected);
+
+        attrs.selected = true;
+        assertTrue(attrs.selected);
+
+        attrs.selected = false;
+        assertFalse(attrs.selected);
+    }
+
+    @Test
+    void testTwoInstancesAreIndependent() {
+        SelectionAttributes attrs1 = new SelectionAttributes(true);
+        SelectionAttributes attrs2 = new SelectionAttributes(false);
+
+        assertNotEquals(attrs1.selected, attrs2.selected);
+
+        attrs2.selected = true;
+        assertTrue(attrs1.selected);
+        assertTrue(attrs2.selected);
+
+        // Mutating one doesn't affect the other
+        attrs1.selected = false;
+        assertFalse(attrs1.selected);
+        assertTrue(attrs2.selected);
+    }
+}