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.Downloaders/UnitFragmentDownloaderBinary.cs

@@ -0,0 +1,74 @@
+using Beam.Abstractions;
+using Beam.Exceptions;
+using Beam.Models;
+using Microsoft.Extensions.Logging;
+
+namespace Beam.Downloaders {
+    /// <summary>
+    /// Groups multiple binary downloads into a single Fragment, applying
+    /// failure detection and exponential-back-off retries for each link.
+    /// </summary>
+    public class UnitFragmentDownloaderBinary<T>
+        : IUnitDownloader<Fragment<Ordered<T>>> {
+        public UnitFragmentDownloaderBinary(HttpClient client,
+                                            AsyncTransformer<ByteDocument, T> transformer,
+                                            AsyncDownloadFailurePredicate<ByteDocument>?[]? failurePredicate = null,
+                                            int fragmentSize = 4,
+                                            ILogger? logger = null,
+                                            IUnitDownloader<T>? internalDownloader = null) {
+            Client = client;
+            Transformer = transformer;
+            FailurePredicate = failurePredicate;
+            UnitDownloader = internalDownloader
+                              ?? new UnitDownloaderBinary<T>(Client, Transformer, FailurePredicate);
+            LinksPerDownload = fragmentSize;
+            Logger = logger;
+        }
+
+        public HttpClient Client { get; }
+        public AsyncTransformer<ByteDocument, T> Transformer { get; }
+        public AsyncDownloadFailurePredicate<ByteDocument>?[]? FailurePredicate { get; }
+        public int LinksPerDownload { get; set; }
+        public ILogger? Logger { get; set; }
+
+        private readonly IUnitDownloader<T> UnitDownloader;
+
+        async Task<(bool, Fragment<Ordered<T>>?)> IUnitDownloader<Fragment<Ordered<T>>>.TryDownload(
+            IOrdered<string>[] link,
+            CancellationToken ct,
+            int maximumRetryCount,
+            IProgress<IRetryReport>? tryProgress) {
+            var fragment = new Fragment<Ordered<T>>(link.Length);
+            if (!Fragment<Ordered<T>>.TryAcquireUpdater(fragment, out var updater))
+                throw new AssertionException(Exceptions.Exceptions.fragment_locked);
+
+            var isFailure = false;
+
+            await Parallel.ForEachAsync(link, async (orderedLink, pct) => {
+                pct.ThrowIfCancellationRequested();
+                ct.ThrowIfCancellationRequested();
+
+                var (success, downloaded) =
+                    await UnitDownloader.TryDownload([orderedLink],
+                                                     ct,
+                                                     maximumRetryCount,
+                                                     tryProgress);
+
+                if (!success || downloaded is null) {
+                    Interlocked.Exchange(ref isFailure, true);
+                    Logger?.LogError("Failed to retrieve {Link} order={Order}",
+                                     orderedLink.Data, orderedLink.Order);
+                    return;
+                }
+
+                updater(new Ordered<T>(downloaded, orderedLink.Order));
+            });
+
+            if (!isFailure)
+                Fragment<Ordered<T>>.SetComplete(fragment, true);
+
+            Fragment<Ordered<T>>.TryReleaseUpdater(fragment, updater);
+            return (!isFailure, fragment);
+        }
+    }
+}