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.IContextStage.cs

@@ -1,4 +1,7 @@
-using Beam.Playwright;
+using Beam.Abstractions;
+using Beam.Downloaders;
+using Beam.Models;
+using Beam.Playwright;
 using Beam.Stealth;
 
 namespace Beam.Fluent {
@@ -7,7 +10,7 @@ public static partial class DownloadBuilder<RawType, OutType> {
             IContextStage Configure(Action<DownloadContextBuilder<RawType>> configure);
             IContextStage WithParallelism(int degree);
             IContextStage WithTimeout(TimeSpan timeout);
-            IContextStage WithRetryReporter(IProgress<RetryReport> reporter);
+            IContextStage WithRetryReporter(IProgress<IRetryReport> reporter);
             DownloadEnumerable<OutType> Build();
             IContextStage UseFragments();
             IContextStage UsePlaywright(PlaywrightAsyncManipulator manipulator);