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/UnitFragmentDownloader.cs

@@ -0,0 +1,62 @@
+using Beam.Abstractions;
+using Beam.Exceptions;
+using Beam.Models;
+using HtmlAgilityPack;
+using Microsoft.Extensions.Logging;
+
+namespace Beam.Downloaders {
+    public class UnitFragmentDownloader<T> : IUnitDownloader<Fragment<Ordered<T>>> {
+        public UnitFragmentDownloader(HtmlWeb web,
+                                      AsyncTransformer<HtmlDocument, T> transformer,
+                                      AsyncDownloadFailurePredicate<HtmlDocument>?[]? failurePredicate = null,
+                                      int fragmentSize = 4,
+                                      ILogger? logger = null,
+                                      IUnitDownloader<T>? internalDownloader = null) {
+            Web = web;
+            Transformer = transformer;
+            FailurePredicate = failurePredicate;
+            UnitDownloader = internalDownloader ?? new UnitDownloader<T>(Web, Transformer, FailurePredicate);
+            LinksPerDownload = fragmentSize;
+            Logger = logger;
+        }
+
+        public HtmlWeb Web { get; }
+        public AsyncTransformer<HtmlDocument, T> Transformer { get; }
+        public AsyncDownloadFailurePredicate<HtmlDocument>?[]? 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) {
+            Fragment<Ordered<T>> fragment = new Fragment<Ordered<T>>(link.Length);
+            if (!Fragment<Ordered<T>>.TryAcquireUpdater(fragment, out var updater))
+                throw new AssertionException(Exceptions.Exceptions.fragment_locked);
+            bool isFailure = false;
+            await Parallel.ForEachAsync(link, async (x, pct) => {
+                pct.ThrowIfCancellationRequested();
+                ct.ThrowIfCancellationRequested();
+                var (result, downloadedT) = await UnitDownloader.TryDownload([x], ct, maximumRetryCount, tryProgress);
+                if (!result) {
+                    Interlocked.Exchange(ref isFailure, true);
+                    Logger?.LogError("Failed to retrieve {0} order={1}", x.Data, x.Order);
+                    return;
+                }
+                if (downloadedT == null) {
+                    Interlocked.Exchange(ref isFailure, true);
+                    Logger?.LogCritical("Failed to retrieve {0} order={1}", x.Data, x.Order);
+                    return;
+                }
+                updater(new Ordered<T>(downloadedT, x.Order));
+            });
+
+            if (!isFailure)
+                Fragment<Ordered<T>>.SetComplete(fragment, true);
+
+            Fragment<Ordered<T>>.TryReleaseUpdater(fragment, updater);
+            
+            return (!isFailure, fragment);
+
+        }
+    }
+}