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.ContextStage.cs

@@ -1,6 +1,10 @@
-using HtmlAgilityPack;
+using Beam.Abstractions;
+using Beam.Models;
+using HtmlAgilityPack;
 using Beam.Playwright;
 using Beam.Stealth;
+using Beam;
+using Beam.Downloaders;
 
 namespace Beam.Fluent {
 public static partial class DownloadBuilder<RawType, OutType> {
@@ -33,7 +37,7 @@ public static partial class DownloadBuilder<RawType, OutType> {
                 return this;
             }
 
-            public IContextStage WithRetryReporter(IProgress<RetryReport> reporter) {
+            public IContextStage WithRetryReporter(IProgress<IRetryReport> reporter) {
                 _ctxBuilder.WithRetryReporter(reporter);
                 return this;
             }
@@ -163,7 +167,7 @@ public static partial class DownloadBuilder<RawType, OutType> {
             }
 
             private IAsyncEnumerator<Ordered<OutType>> ConstructDownloader(DownloadContext<RawType> context) {
-                var copyOfContext = context.CreateBuilder().Build();
+                var copyOfContext = DownloadContextBuilder<RawType>.FromContext(context).Build();
                 return _useFragments switch {
                     true => new SequentialFragmentDownloader<RawType, OutType>(
                         copyOfContext,