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.Data/Template.cs

@@ -0,0 +1,19 @@
+using aeqw89.DataKeys;
+using Beam.Abstractions;
+using Beam.Dynamic;
+
+namespace Beam.Data {
+    public record class Template : ITemplate {
+        public required DataKey<ITemplate> Key { get; set; }
+        public required StateChangerFactory Factory { get; set; }
+        IStateChangerFactory ITemplate.Factory {
+            get => Factory;
+            set => Factory = (StateChangerFactory)value;
+        }
+        public required LinkBuilder Builder { get; set; }
+        ILinkBuilder ITemplate.Builder {
+            get => Builder;
+            set => Builder = (LinkBuilder)value;
+        }
+    }
+}