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.LinkStage.cs

@@ -1,10 +1,11 @@
-using Beam.Models;
+using Beam.Data;
+using Beam.Downloaders;
+using Beam.Dynamic;
+using Beam.Models;
 
 namespace Beam.Fluent {
     public static partial class DownloadBuilder<RawType, OutType> {
         /* ──────────────────────────── Stage types ─────────────────────────── */
-
-
         private sealed record LinkStage(
             WebResource Source,
             State Initial,
@@ -22,7 +23,7 @@ namespace Beam.Fluent {
 
             public ITransformStage WithLinkGenerator() {
                 var template = Data.Templates[Source.Key];
-                var generator = SourceLinkEnumerable.FromGenerator(new OrderedSourceLinkGenerator(
+                var generator = StringEnumerable.FromGenerator(new OrderedLinkGenerator(
                     template.Builder,
                     new NumberedStateChanger(template.Factory.Behavior),
                     Initial, endState));
@@ -31,7 +32,7 @@ namespace Beam.Fluent {
                 return new TransformStage(Source, Data, CtxBuilder);
             }
 
-            public IAlternativeTransformStage WithLinks(IEnumerable<SourceLink> links) {
+            public IAlternativeTransformStage WithLinks(IEnumerable<string> links) {
                 CtxBuilder.WithLinks(links);
                 return new TransformStage(Source, Data, CtxBuilder);
             }