refactor: modularize Beam into new projects and interfaces

- Introduced modularity by splitting Beam into new projects: Beam.Abstractions, Beam.Models, and Beam.Downloaders.
- Refactored existing classes into appropriate namespaces and projects.
- Replaced specific implementations with abstractions (e.g., SourceLinkBuilder to LinkBuilder, State to IState, etc.).
- Updated interfaces: added ITemplate, IArticleData, IDownloadReport, and others for improved extensibility.
- Removed deprecated classes like SourceLinkBuilder and StateChangerFactory.
- Enhanced link handling in downloaders by refactoring to use `string` over `SourceLink`.
- Consolidated shared logic under Beam.Abstractions.

Beam.Stealth/StealthUnitDownloader.cs

@@ -6,8 +6,12 @@ using System.Diagnostics;
 using System.Linq;
 using System.Text;
 using System.Threading.Tasks;
+using Beam.Downloaders;
+using Beam.Models;
 
 namespace Beam.Stealth {
+    using File = System.IO.File;
+    
     public class StealthUnitDownloader<T> : UnitDownloaderBinary<T> {
         public StealthConfig Config { get; }
         public StealthAsyncManipulator Manipulator { get; }