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.Models/Beam.Models.csproj

@@ -7,11 +7,11 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <ProjectReference Include="..\Beam.Dynamic\Beam.Dynamic.csproj" />
-    <ProjectReference Include="..\Beam\Beam.csproj" />
+    <ProjectReference Include="..\Beam.Abstractions\Beam.Abstractions.csproj" />
   </ItemGroup>
 
   <ItemGroup>
+    <PackageReference Include="aeqw89.PersistentData" Version="1.3.3" />
     <PackageReference Include="EntityFramework" Version="6.5.1" />
     <PackageReference Include="Microsoft.EntityFrameworkCore.Design" Version="9.0.8">
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
@@ -23,4 +23,9 @@
     </PackageReference>
   </ItemGroup>
 
+  <ItemGroup>
+    <Folder Include="Abstract\" />
+    <Folder Include="Closed Concrete\" />
+  </ItemGroup>
+
 </Project>