Fix NavigateAndReset navigation for Maui (#4076)

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**What kind of change does this PR introduce?**
<!-- Bug fix, feature, docs update, ... -->

Fix
closes #3820 

**What is the current behavior?**
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#3820 

**What is the new behavior?**
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Changed RoutedViewHost to be a partial class to allow splitting its
implementation.
Refactored navigation stack comparison to use the StacksAreDifferent()
method for improved clarity and maintainability.

**What might this PR break?**

If currently calling NavigateAndReset at startup may cause navigation to
end up in a different place.

**Please check if the PR fulfills these requirements**
- [ ] Tests for the changes have been added (for bug fixes / features)
- [ ] Docs have been added / updated (for bug fixes / features)

**Other information**:

Author: ChrisPulman

src/ReactiveUI.Maui/RoutedViewHost.cs

@@ -15,7 +15,7 @@ namespace ReactiveUI.Maui;
 /// </summary>
 /// <seealso cref="NavigationPage" />
 /// <seealso cref="IActivatableView" />
-public class RoutedViewHost : NavigationPage, IActivatableView, IEnableLogger
+public partial class RoutedViewHost : NavigationPage, IActivatableView, IEnableLogger
 {
     /// <summary>
     /// The router bindable property.
@@ -76,7 +76,7 @@ public RoutedViewHost()
 
             Router?
                 .Navigate
-                .Where(_ => Navigation.NavigationStack.Count != Router.NavigationStack.Count)
+                .Where(_ => StacksAreDifferent())
                 .ObserveOn(RxApp.MainThreadScheduler)
                 .SelectMany(_ => PagesForViewModel(Router.GetCurrentViewModel()))
                 .SelectMany(async page =>