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/UnitDownloaderBinary.cs

@@ -0,0 +1,74 @@
+using Beam.Abstractions;
+using Beam.Models;
+
+namespace Beam.Downloaders {
+    /// <summary>
+    /// A download-managing class that retrieves binary data through <see cref="HttpClient"/>,
+    /// applies an <see cref="AsyncBinaryTransformer{T}"/>, and supports failure detection
+    /// plus exponential-back-off retries. Safe to instantiate per request.
+    /// </summary>
+    public class UnitDownloaderBinary<T>(
+        HttpClient client,
+        AsyncTransformer<ByteDocument, T> transformer,
+        AsyncDownloadFailurePredicate<ByteDocument>?[]? failurePredicates = null)
+        : IUnitDownloader<T> {
+        public HttpClient Client { get; } = client;
+        public virtual AsyncTransformer<ByteDocument, T> Transformer { get; } = transformer;
+        public virtual AsyncDownloadFailurePredicate<ByteDocument>?[]? FailurePredicates { get; } = failurePredicates;
+
+        public int LinksPerDownload { get; } = 1;
+
+        /// <summary>Runs all configured failure predicates in parallel on the raw HTTP response.</summary>
+        protected virtual async Task<bool> IsFailure(ByteDocument response) {
+            if (FailurePredicates is null) return false;
+
+            var failed = false;
+            await Parallel.ForEachAsync(FailurePredicates, async (pred, ct) => {
+                if (failed || pred is null) return;
+                if (await pred(response))
+                    failed = true;
+            });
+            return failed;
+        }
+
+        /// <summary>One attempt without retries or back-off.</summary>
+        protected virtual async Task<(bool Success, T? Result)> TryDownloadWithNoRetries(string link, CancellationToken ct) {
+            try {
+                using var response = await Client.GetAsync(link, HttpCompletionOption.ResponseHeadersRead, ct);
+                if (!response.IsSuccessStatusCode) return (false, default);
+
+                var bytes = await response.Content.ReadAsByteArrayAsync(ct);
+                var doc = new ByteDocument(link, bytes);
+                if (await IsFailure(doc)) return (false, default);
+
+                return (true, await Transformer(doc));
+            } catch {
+                return (false, default);
+            }
+        }
+
+        public async Task<(bool, T?)> TryDownload(
+            IOrdered<string>[] link,
+            CancellationToken ct,
+            int maximumRetryCount = 7,
+            IProgress<IRetryReport>? tryProgress = null) {
+            if (link.Length == 0) return (false, default);
+
+            T? result = default;
+            var attempt = 0;
+
+            while (attempt < maximumRetryCount) {
+                ct.ThrowIfCancellationRequested();
+
+                (var success, result) = await TryDownloadWithNoRetries(link[0].Data, ct);
+                if (success && result is not null) return (true, result);
+
+                ++attempt;
+                tryProgress?.Report(new RetryReport(attempt, link[0].Data));
+                await Task.Delay((int)Math.Pow(2, attempt) * 1000, ct);
+            }
+
+            return (false, result);
+        }
+    }
+}