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.Api/ApiCalls.cs

@@ -0,0 +1,48 @@
+// ApiCalls.cs
+using System.Collections.Concurrent;
+using System.Net;
+using Microsoft.Extensions.Logging;
+
+namespace Beam {
+    /// <summary>
+    /// Executes a batch of <see cref="ApiCall"/>s using either sequential or parallel strategy.
+    /// </summary>
+    public sealed class ApiCalls {
+        private readonly IReadOnlyList<ApiCall> _calls;
+        private readonly int _maxDegree;
+
+        internal ApiCalls(IReadOnlyList<ApiCall> calls, int? maxDegree) {
+            _calls = calls ?? throw new ArgumentNullException(nameof(calls));
+            _maxDegree = Math.Max(1, maxDegree ?? 1);
+        }
+
+        /// <summary>
+        /// Runs every call and returns the ordered list of <see cref="ApiResponse"/>s.
+        /// </summary>
+        public async Task<IReadOnlyList<ApiResponse>> ExecuteAsync(
+            ILogger<ApiResponse>? logger = null,
+            (int @try, int max)? tries = null,
+            CancellationToken ct = default) {
+            if (_maxDegree == 1) {
+                // sequential
+                var sequential = new List<ApiResponse>(_calls.Count);
+                foreach (var call in _calls)
+                    sequential.Add(await call.GetResponse(logger, tries, ct));
+                return sequential;
+            }
+
+            // parallel
+            var bag = new ConcurrentBag<(int idx, ApiResponse res)>();
+            await Parallel.ForEachAsync(
+                _calls.Select((c, i) => (call: c, idx: i)),
+                new ParallelOptions { MaxDegreeOfParallelism = _maxDegree, CancellationToken = ct },
+                async (item, token) => {
+                    var response = await item.call.GetResponse(logger, tries, token);
+                    bag.Add((item.idx, response));
+                });
+
+            // keep original ordering
+            return bag.OrderBy(x => x.idx).Select(x => x.res).ToList();
+        }
+    }
+}