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/WebResource.cs

@@ -0,0 +1,29 @@
+using aeqw89.DataKeys;
+using Beam.Dynamic;
+
+namespace Beam.Data {
+    /// <summary>
+    /// Represents a specific resource accessible online; e.g. a book's contents.
+    /// </summary>
+    /// <remarks>
+    /// A resource should be one-to-one with a web request. 
+    /// </remarks>
+    /// <param name="key"></param>
+    public class WebResource(DataKey<WebResource> key) : IKeyed<WebResource> {
+        public DataKey<WebResource> Key { get; set; } = key;
+
+        public required DataKey<DataBindings> Bindings { get; set; }
+        public string? Name { get; set; }
+        public string? Domain { get; set; }
+        public string? Description { get; set; }
+
+
+        public WebResource() : this(new(string.Empty)) { }
+
+        //public Entity ToRecord(BeamDataDictionary sdd) {
+        //    return new Entity(this, sdd.Bindings[Bindings]);
+        //}
+
+        //public record class Entity(WebResource Resource, DataBindings Bindings);
+    }
+}