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/Fragment.cs

@@ -0,0 +1,37 @@
+using System.Collections.Concurrent;
+using System.Diagnostics.CodeAnalysis;
+
+namespace Beam.Models {
+    public sealed class Fragment<T>(int size) {
+        public int Size => FragmentBag.Count;
+        public int MaxSize { get; } = size;
+        private ConcurrentBag<T> FragmentBag { get; set; } = new();
+        public bool TryTake([NotNullWhen(true)] out T? shard) {
+            return FragmentBag.TryTake(out shard) && shard != null;
+        }
+
+        private bool? Complete = false;
+        public bool IsComplete => Complete ?? Size == MaxSize;
+
+        private bool UpdaterLocked = false;
+
+        public static bool TryAcquireUpdater(Fragment<T> fragment, [NotNullWhen(true)] out Action<T>? updater) {
+            updater = null;
+            if (Interlocked.CompareExchange(ref fragment.UpdaterLocked, true, false) == true) 
+                // equivalent to : fragment.UpdaterLocked == true, side-effect: sets fragment.UpdaterLocked to true
+                return false;
+            updater = fragment.FragmentBag.Add;
+            return true;
+        }
+        public static bool TryReleaseUpdater(Fragment<T> fragment, Action<T> updater) {
+            if (updater == fragment.FragmentBag.Add) {
+                Interlocked.Exchange(ref fragment.UpdaterLocked, false);
+                return true;
+            }
+            return false;
+        }
+        public static void SetComplete(Fragment<T> fragment, bool status) {
+            fragment.Complete = status;
+        }
+    }
+}