Move hardcoded error strings in RemoveRegisterAttribute and GenerateCompressedAssembliesNativeSourceFiles to Properties.Resources

[Copilot]

Two MSBuild tasks used Log.LogError with hardcoded, non-localized strings and no XA error codes. This replaces them with Log.LogCodedError using Properties.Resources entries.

Changes

• New error codes XA2024 / XA2025 added to Resources.resx and Resources.Designer.cs
• RemoveRegisterAttribute.cs and GenerateCompressedAssembliesNativeSourceFiles.cs — replaced Log.LogError with Log.LogCodedError
• Documentation — added xa2024.md, xa2025.md, and updated index.md

// Before
Log.LogError ($"Unable to load assembly '{mono_android}'");
Log.LogError ($"Assembly {assembly.ItemSpec} does not exist");

// After
Log.LogCodedError ("XA2024", Properties.Resources.XA2024, mono_android);
Log.LogCodedError ("XA2025", Properties.Resources.XA2025, assembly.ItemSpec);