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.Models/ImmutableState.cs

@@ -0,0 +1,26 @@
+using System.Text.Json.Serialization;
+
+namespace Beam.Models {
+    public readonly struct ImmutableState {
+        readonly string[] state;
+
+        [JsonConstructor]
+        public ImmutableState(string[] state) {
+            this.state = state ?? [];
+        }
+
+        public string[] State => state ?? [];
+
+        public readonly Span<string> AsSpan() => state ?? [];
+
+        public readonly State Copy()
+            => new((string[])(state ?? []).Clone());
+
+        public readonly string this[Index i] {
+            get => state[i];
+        }
+
+        public static implicit operator State(ImmutableState state)
+            => state.Copy();
+    }
+}