WMC9999: MissingManifestResourceException when building WinUI 3 app with dotnet msbuild on non-English Windows

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## Description

Building a WinUI 3 app using `dotnet msbuild` fails with `WMC9999` on non-English
(e.g. Russian) Windows systems. The root cause is a resource name mismatch in
`XamlCompiler.exe` shipped in the `microsoft.windowsappsdk.winui` NuGet package.

## Steps to Reproduce

1. Windows with non-English UI language (tested: Russian / ru-RU)
2. Install .NET 10 SDK
3. Build any WinUI 3 project using `dotnet msbuild` (MSBuildRuntimeType=Core)

## Error

Xaml Internal Error error WMC9999: Could not find any resources appropriate for
the specified culture or the neutral culture. Make sure
"Microsoft.UI.Xaml.Markup.Compiler.ErrorMessages.resources" was correctly
embedded or linked into assembly "XamlCompiler" at compile time.

## Root Cause

In `microsoft.windowsappsdk.winui 2.1.0`, `XamlCompiler.exe` contains class
`Microsoft.UI.Xaml.Markup.Compiler.ErrorMessages` whose `ResourceManager` looks
for resource `Microsoft.UI.Xaml.Markup.Compiler.ErrorMessages.resources` in the
XamlCompiler assembly. However, the embedded resource is still named with the
**old namespace**: `Microsoft.Windows.UI.Xaml.Build.Tasks.ErrorMessages.resources`.

This mismatch is masked on English Windows because `NeutralResourcesLanguageAttribute`
is absent and .NET Framework's neutral-culture fallback silently swallows the error
in that context. On non-English systems the `MissingManifestResourceException`
surfaces as WMC9999.

Note: this only affects `dotnet msbuild` (Core runtime), which uses `XamlCompiler.exe`
as a subprocess (`UseXamlCompilerExecutable=true`). Full Visual Studio is unaffected
because it uses its own XAML compiler host.

## Fix

Two changes needed in `XamlCompiler.exe`:
1. Rename embedded resource from `Microsoft.Windows.UI.Xaml.Build.Tasks.ErrorMessages`
→ `Microsoft.UI.Xaml.Markup.Compiler.ErrorMessages`
2. Add `[assembly: NeutralResourcesLanguage("en-US")]`

## Environment

- Package: `microsoft.windowsappsdk.winui` 2.1.0
- OS: Windows 11 ru-RU
- SDK: .NET 10.0.300
- Build: `dotnet msbuild` (MSBuild 18.6.3)

Start by locating XamlCompiler.exe in the microsoft.windowsappsdk.winui package and inspect its embedded resource name and assembly attributes. Reproduce with dotnet msbuild on a non-English Windows system; done means the resource resolves and the WinUI build no longer reports WMC9999.

Written by the indexing model from the issue text.