Add comprehensive external change equality tests

Author: fatbobman

Tests/ObservableDefaultsTests/ExternalChangeEqualityTests.swift

@@ -23,4 +23,45 @@ struct ExternalChangeEqualityTests {
         tracking(model, \.age, .userDefaults, false)
         userDefaults.set("NewName", forKey: "name")
     }
+
+    @Test("Same value write does not notify for backed property")
+    func sameValueWriteNotNotified() {
+        let userDefaults = UserDefaults.getTestInstance(suiteName: #function)
+        let model = MockModel(userDefaults: userDefaults)
+
+        // Track name - expect NO mutation when writing the same value
+        tracking(model, \.name, .userDefaults, false)
+        userDefaults.set("Test", forKey: "name")
+    }
+
+    @MainActor
+    @Test("MainActor unrelated property not notified when another property changes externally")
+    func mainActorUnrelatedPropertyNotNotified() async {
+        let userDefaults = UserDefaults.getTestInstance(suiteName: #function)
+        let model = MockModelMainActor(userDefaults: userDefaults)
+
+        // Track count - expect NO mutation when only name changes
+        tracking(model, \.count, .userDefaults, false)
+        userDefaults.set("ExternalMainActorName", forKey: "name")
+
+        // MainActor observer path uses .main queue
+        try? await Task.sleep(nanoseconds: 100_000_000)
+        #expect(model.name == "ExternalMainActorName")
+        #expect(model.count == 0)
+    }
+
+    @MainActor
+    @Test("MainActor same value write does not notify for backed property")
+    func mainActorSameValueWriteNotNotified() async {
+        let userDefaults = UserDefaults.getTestInstance(suiteName: #function)
+        let model = MockModelMainActor(userDefaults: userDefaults)
+
+        // Track name - expect NO mutation when writing the same value
+        tracking(model, \.name, .userDefaults, false)
+        userDefaults.set("Test", forKey: "name")
+
+        // MainActor observer path uses .main queue
+        try? await Task.sleep(nanoseconds: 100_000_000)
+        #expect(model.name == "Test")
+    }
 }