Exclude ObservableOnly properties from notification handler

In observeFirst mode, filter out @ObservableOnly properties from the
didChangeNotification handler since they are not stored in UserDefaults
and cannot be triggered by UserDefaults changes. This simplifies the
generated code and prevents dead case branches.

Use compactMap during metas construction to exclude non-backed properties,
removing the need for the isBacked discriminator in caseCode.

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

Author: gfreezy

Sources/ObservableDefaultsMacros/Macros/ObservableDefaultsMacro.swift

@@ -148,9 +148,15 @@ extension ObservableDefaultsMacros: MemberMacro {
                 return varDecl
             }
 
-        // Build mapping between properties and their UserDefaults keys
-        let metas: [(userDefaultsKey: String, propertyID: String, isBacked: Bool)] = persistentProperties
-            .map { property in
+        // Build mapping between properties and their UserDefaults keys.
+        // In observeFirst mode, only explicitly @DefaultsBacked properties are backed;
+        // non-backed (ObservableOnly) properties are excluded because they are not
+        // stored in UserDefaults and should not appear in the notification handler.
+        let metas: [(userDefaultsKey: String, propertyID: String)] = persistentProperties
+            .compactMap { property in
+                if observeFirst, !property.hasAttribute(named: DefaultsBackedMacro.name) {
+                    return nil
+                }
                 let key =
                     property.attributes.extractValue(
                         forAttribute: DefaultsBackedMacro.name,
@@ -159,10 +165,7 @@ extension ObservableDefaultsMacros: MemberMacro {
                         forAttribute: DefaultsKeyMacro.name,
                         argument: DefaultsKeyMacro.key) ?? property.identifier?.text ?? ""
                 let propertyID = property.identifier?.text ?? ""
-                // In observeFirst mode, only explicitly @DefaultsBacked properties are backed;
-                // otherwise all persistent properties are backed.
-                let isBacked = !observeFirst || property.hasAttribute(named: DefaultsBackedMacro.name)
-                return (key, propertyID, isBacked)
+                return (key, propertyID)
             }
 
         // Generate keyPath mapping for external change handling
@@ -181,22 +184,7 @@ extension ObservableDefaultsMacros: MemberMacro {
         let caseCode = metas.enumerated().map { index, meta in
             let caseIndent = index == 0 ? "" : "                "
             // swiftformat:disable all
-            if !meta.isBacked {
-                // ObservableOnly properties are not stored in UserDefaults,
-                // so we cannot compare values — keep the original behavior.
-                if hasMainActor {
-                    return """
-                        \(caseIndent)case prefix + "\(meta.userDefaultsKey)":
-                        \(caseIndent)    MainActor.assumeIsolated {
-                        \(caseIndent)        host._$observationRegistrar.withMutation(of: host, keyPath: \\.\(meta.propertyID)) {}
-                        \(caseIndent)    }
-                        """
-                } else {
-                    return """
-                        \(caseIndent)case prefix + "\(meta.userDefaultsKey)": host._$observationRegistrar.withMutation(of: host, keyPath: \\.\(meta.propertyID)) {}
-                        """
-                }
-            } else if hasMainActor {
+            if hasMainActor {
                 return """
                     \(caseIndent)case prefix + "\(meta.userDefaultsKey)":
                     \(caseIndent)    MainActor.assumeIsolated {