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.Fluent/DownloadBuilder.cs

@@ -3,6 +3,8 @@ using Beam;
 using Microsoft.Extensions.Logging;
 using System;
 using System.Collections.Generic;
+using Beam.Data;
+using Beam.Downloaders;
 using Beam.Models;
 
 namespace Beam.Fluent {
@@ -22,7 +24,7 @@ namespace Beam.Fluent {
 
         private static ILinkStage Create(DataKey<ResourceDictionary> resourceDict, BeamDataContext data, string kind) {
             var (source, initial) = Resolve(resourceDict, kind, data);
-            var ctxBuilder = new DownloadContextBuilder<RawType>().WithLinks(Array.Empty<SourceLink>()); // placeholder, filled later.
+            var ctxBuilder = new DownloadContextBuilder<RawType>().WithLinks([]); // placeholder, filled later.
             return new LinkStage(source, initial, data, ctxBuilder);
         }