Feature Introduce ReactiveUIBuilder (#4086)

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[Contribute](https://github.com/reactiveui/reactiveui#contribute)
section of the README -->

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

Feature

**What is the current behavior?**
<!-- You can also link to an open issue here. -->

Application is magically wired up based on packages using reflection

**What is the new behavior?**
<!-- If this is a feature change -->

Aims to add the ReactiveUIBuilder pattern and related platform extension
methods for configuring ReactiveUI services without reflection,
supporting AOT environments. Includes core builder implementation,
platform-specific extension methods (WPF, WinForms, MAUI, WinUI, Blazor,
Drawing), interface for custom modules, and comprehensive tests.

Updates API approval files to reflect new public APIs.

**What might this PR break?**

This is intended to be a Parallel option for end users so none expected

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

**Other information**:

Author: ChrisPulman

src/Directory.Packages.props

@@ -4,7 +4,7 @@
     <CentralPackageTransitivePinningEnabled>true</CentralPackageTransitivePinningEnabled>
   </PropertyGroup>
   <PropertyGroup>
-    <SplatVersion>15.4.1</SplatVersion>
+    <SplatVersion>15.5.3</SplatVersion>
     <XamarinAndroidXCoreVersion>1.13.1.4</XamarinAndroidXCoreVersion>
     <XamarinAndroidXLifecycleLiveDataVersion>2.8.4.1</XamarinAndroidXLifecycleLiveDataVersion>
   </PropertyGroup>

src/ReactiveUI.AOTTests/AOTCompatibilityTests.cs

@@ -4,10 +4,7 @@
 // See the LICENSE file in the project root for full license information.
 
 using System.Diagnostics.CodeAnalysis;
-using System.Reactive;
 using System.Reactive.Linq;
-using ReactiveUI;
-using Xunit;
 
 namespace ReactiveUI.AOTTests;
 

src/ReactiveUI.AOTTests/AdvancedAOTTests.cs

@@ -4,13 +4,10 @@
 // See the LICENSE file in the project root for full license information.
 
 using System.Diagnostics.CodeAnalysis;
-using System.Reactive;
 using System.Reactive.Disposables;
 using System.Reactive.Linq;
 using System.Reactive.Subjects;
-using ReactiveUI;
 using Splat;
-using Xunit;
 
 namespace ReactiveUI.AOTTests;
 

src/ReactiveUI.AOTTests/ComprehensiveAOTMarkupTests.cs

@@ -4,14 +4,11 @@
 // See the LICENSE file in the project root for full license information.
 
 using System.Diagnostics.CodeAnalysis;
-using System.Reactive;
 using System.Reactive.Concurrency;
 using System.Reactive.Disposables;
 using System.Reactive.Linq;
 using System.Reactive.Subjects;
-using ReactiveUI;
 using Splat;
-using Xunit;
 
 namespace ReactiveUI.AOTTests;
 

src/ReactiveUI.AOTTests/ComprehensiveAOTTests.cs

@@ -4,14 +4,11 @@
 // See the LICENSE file in the project root for full license information.
 
 using System.Diagnostics.CodeAnalysis;
-using System.Reactive;
 using System.Reactive.Concurrency;
 using System.Reactive.Disposables;
 using System.Reactive.Linq;
 using System.Reactive.Subjects;
-using ReactiveUI;
 using Splat;
-using Xunit;
 
 namespace ReactiveUI.AOTTests;
 

src/ReactiveUI.AOTTests/FinalAOTValidationTests.cs

@@ -9,9 +9,7 @@
 using System.Reactive.Disposables;
 using System.Reactive.Linq;
 using System.Reactive.Subjects;
-using ReactiveUI;
 using Splat;
-using Xunit;
 
 namespace ReactiveUI.AOTTests;
 

src/ReactiveUI.AOTTests/TestActivatableViewModel.cs

@@ -3,8 +3,6 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for full license information.
 
-using ReactiveUI;
-
 namespace ReactiveUI.AOTTests;
 
 /// <summary>

src/ReactiveUI.AOTTests/TestReactiveObject.cs

@@ -5,7 +5,6 @@
 
 using System.Diagnostics.CodeAnalysis;
 using System.Reactive.Linq;
-using ReactiveUI;
 
 namespace ReactiveUI.AOTTests;
 

src/ReactiveUI.AOTTests/TestRoutableViewModel.cs

@@ -3,8 +3,6 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for full license information.
 
-using ReactiveUI;
-
 namespace ReactiveUI.AOTTests;
 
 /// <summary>

src/ReactiveUI.AndroidX/ReactiveAppCompatActivity.cs

@@ -3,7 +3,6 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for full license information.
 
-using System.Diagnostics.CodeAnalysis;
 using Android.App;
 using Android.Content;
 using Android.Runtime;