Fix DynamicStreamWidget to use watchValue instead of constructor param

The previous example used a constructor parameter with WatchingWidget,
which wouldn't properly re-evaluate when the parameter changes. Now
uses watchValue to get a dynamic value from the manager, which is a
realistic use case for allowStreamChange: true.

Changes:
- Added useStream1 ValueNotifier to StreamManager
- Updated DynamicStreamWidget to use watchValue to get the selection
- Removed constructor parameter that wouldn't work correctly

Author: escamoteur

code_samples/lib/watch_it/_shared/stubs.dart

@@ -627,9 +627,13 @@ class StreamManager {
   Stream<int> get stream1 => _controller1.stream;
   Stream<int> get stream2 => _controller2.stream;
 
+  // Controls which stream to use
+  final useStream1 = ValueNotifier<bool>(true);
+
   void dispose() {
     _controller1.close();
     _controller2.close();
+    useStream1.dispose();
   }
 }
 

code_samples/lib/watch_it/multiple_values_inline_combine_safe.dart

@@ -79,14 +79,15 @@ class UnsafeInlineCombineWidget extends WatchingWidget {
 
 // #region when_to_use_allow_change
 class DynamicStreamWidget extends WatchingWidget {
-  const DynamicStreamWidget({super.key, required this.useStream1});
-
-  final bool useStream1;
+  const DynamicStreamWidget({super.key});
 
   @override
   Widget build(BuildContext context) {
-    // ✅ CORRECT use of allowObservableChange: true
-    // Stream actually changes based on widget parameter
+    // Get dynamic value that determines which stream to watch
+    final useStream1 = watchValue((StreamManager m) => m.useStream1);
+
+    // ✅ CORRECT use of allowStreamChange: true
+    // Stream identity changes when useStream1 changes
     final data = watchStream(
       (StreamManager m) => useStream1 ? m.stream1 : m.stream2,
       initialValue: 0,