Fix getters mutating state on repeated access

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/stores/conformance.test.js

@@ -196,5 +196,18 @@ describe('conformanceStore', () => {
       // The date attribute should show minutes (30), not month (06)
       expect(result[0].attributes[0].value).toMatch(/:30:/);
     });
+
+    it('cases getter does not corrupt original state on repeated access', () => {
+      const originalDate = '2023-06-15T10:30:00Z';
+      store.allCases = [{
+        started_at: originalDate,
+        completed_at: '2023-06-15T11:45:00Z',
+        facets: [],
+        attributes: [],
+      }];
+      store.cases;
+      // Original state should still contain the ISO date
+      expect(store.allCases[0].started_at).toBe(originalDate);
+    });
   });
 });