Fix external change handler to only notify on actual value changes

When UserDefaults.didChangeNotification fires, the handler now reads the current
value and compares it with the cached value using shouldSetValue() before triggering
the mutation notification. This prevents unnecessary SwiftUI view re-evaluations
when unrelated properties change in UserDefaults.

Also add test to verify unrelated properties are not notified on external changes.

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

Author: gfreezy

Sources/ObservableDefaultsMacros/Macros/ObservableDefaultsMacro.swift

@@ -182,12 +182,21 @@ extension ObservableDefaultsMacros: MemberMacro {
                 return """
                     \(caseIndent)case prefix + "\(meta.userDefaultsKey)":
                     \(caseIndent)    MainActor.assumeIsolated {
-                    \(caseIndent)        host._$observationRegistrar.withMutation(of: host, keyPath: \\.\(meta.propertyID)) {}
+                    \(caseIndent)        let newValue = UserDefaultsWrapper.getValue(fullKey, host._\(meta.propertyID), host._userDefaults)
+                    \(caseIndent)        if host.shouldSetValue(newValue, host._\(meta.propertyID)) {
+                    \(caseIndent)            host._\(meta.propertyID) = newValue
+                    \(caseIndent)            host._$observationRegistrar.withMutation(of: host, keyPath: \\.\(meta.propertyID)) {}
+                    \(caseIndent)        }
                     \(caseIndent)    }
                     """
             } else {
                 return """
-                    \(caseIndent)case prefix + "\(meta.userDefaultsKey)": host._$observationRegistrar.withMutation(of: host, keyPath: \\.\(meta.propertyID)) {}
+                    \(caseIndent)case prefix + "\(meta.userDefaultsKey)":
+                    \(caseIndent)    let newValue = UserDefaultsWrapper.getValue(fullKey, host._\(meta.propertyID), host._userDefaults)
+                    \(caseIndent)    if host.shouldSetValue(newValue, host._\(meta.propertyID)) {
+                    \(caseIndent)        host._\(meta.propertyID) = newValue
+                    \(caseIndent)        host._$observationRegistrar.withMutation(of: host, keyPath: \\.\(meta.propertyID)) {}
+                    \(caseIndent)    }
                     """
             }
             // swiftformat:enable all

Tests/ObservableDefaultsTests/ExternalChangeEqualityTests.swift

@@ -0,0 +1,26 @@
+//
+//  ExternalChangeEqualityTests.swift
+//  ObservableDefaults
+//
+//  Tests that the external UserDefaults change notification handler
+//  only triggers observation mutations when values actually change.
+//
+
+import Foundation
+import ObservableDefaults
+import Observation
+import Testing
+
+@Suite("External Change Equality Check")
+struct ExternalChangeEqualityTests {
+
+    @Test("Unrelated property not notified when another property changes externally")
+    func unrelatedPropertyNotNotified() {
+        let userDefaults = UserDefaults.getTestInstance(suiteName: #function)
+        let model = MockModel(userDefaults: userDefaults)
+
+        // Track age - expect NO mutation when only name changes
+        tracking(model, \.age, .userDefaults, false)
+        userDefaults.set("NewName", forKey: "name")
+    }
+}