Expose new XML namespace for internal types (#4031)

* Initial plan

* Add materialDesignInternal XML namespace and update README documentation

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Co-authored-by: nicolaihenriksen <19572699+nicolaihenriksen@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: nicolaihenriksen <19572699+nicolaihenriksen@users.noreply.github.com>

Author: Copilot

README.md

@@ -83,6 +83,25 @@ Install-Package MaterialDesignThemes
   [...] >
 ```
 
+### XML Namespaces
+
+The toolkit exposes two XML namespaces for use in XAML:
+
+| Prefix | URI | Description |
+|--------|-----|-------------|
+| `materialDesign` | `http://materialdesigninxaml.net/winfx/xaml/themes` | Primary namespace for all public Material Design types, controls, and converters. |
+| `materialDesignInternal` | `http://materialdesigninxaml.net/winfx/xaml/themes-internal` | Namespace for types in internal sub-namespaces (`MaterialDesignThemes.Wpf.Internal`, `MaterialDesignThemes.Wpf.Converters.Internal`, `MaterialDesignThemes.Wpf.Behaviors.Internal`). |
+
+> [!WARNING]
+> Types exposed via the `materialDesignInternal` namespace are considered **internal implementation details** and are **not guaranteed to maintain backwards-compatibility between versions**. Use them at your own risk.
+
+To use the internal namespace in XAML:
+
+```xml
+<Window ...
+  xmlns:materialDesignInternal="http://materialdesigninxaml.net/winfx/xaml/themes-internal">
+```
+
 
 ## Building the source
 

src/MaterialDesignThemes.Wpf/Properties/AssemblyInfo.cs

@@ -5,6 +5,11 @@
 [assembly: XmlnsDefinition("http://materialdesigninxaml.net/winfx/xaml/themes", "MaterialDesignThemes.Wpf.Transitions")]
 [assembly: XmlnsDefinition("http://materialdesigninxaml.net/winfx/xaml/themes", "MaterialDesignThemes.Wpf.Converters")]
 
+[assembly: XmlnsPrefix("http://materialdesigninxaml.net/winfx/xaml/themes-internal", "materialDesignInternal")]
+[assembly: XmlnsDefinition("http://materialdesigninxaml.net/winfx/xaml/themes-internal", "MaterialDesignThemes.Wpf.Internal")]
+[assembly: XmlnsDefinition("http://materialdesigninxaml.net/winfx/xaml/themes-internal", "MaterialDesignThemes.Wpf.Converters.Internal")]
+[assembly: XmlnsDefinition("http://materialdesigninxaml.net/winfx/xaml/themes-internal", "MaterialDesignThemes.Wpf.Behaviors.Internal")]
+
 // Setting ComVisible to false makes the types in this assembly not visible 
 // to COM components.  If you need to access a type in this assembly from 
 // COM, set the ComVisible attribute to true on that type.