fix(Foundation): implement valueForKey: for CPSet KVC compliance

Resolves testKVCCountSetOperators failure in CPSetTest. The previous implementation lacked an override for valueForKey:, causing the runtime to fall back to the default CPObject behavior, which failed to recognize KVC collection operators (e.g., @count).

Delegating valueForKey: to valueForKeyPath: ensures that CPSet correctly routes KVC operations through the existing collection operator logic.

Foundation/CPSet/CPMutableSet.j

@@ -226,6 +226,14 @@
 	}
 }
 
+- (id)valueForKey:(CPString)aKey
+{
+	// If the key starts with @, it is an operator path.
+	// If not, it is a property path that we want applied to all members.
+	// In either case, valueForKeyPath: handles both scenarios correctly.
+	return [self valueForKeyPath:aKey];
+}
+
 - (void)setValue:(id)aValue forKey:(CPString)aKey
 {
 	var containedObject,