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/ApiCallsBuilder.cs

@@ -0,0 +1,47 @@
+// ApiCallsBuilder.cs
+using System.Net;
+
+namespace Beam {
+    /// <summary>
+    /// Fluent builder for <see cref="ApiCalls"/>.
+    /// </summary>
+    public sealed class ApiCallsBuilder {
+        private readonly List<ApiCall> _calls = [];
+        private int? _parallelism = 1; // default = sequential
+
+        public ApiCallsBuilder Add(ApiCall call) {
+            _calls.Add(call ?? throw new ArgumentNullException(nameof(call)));
+            return this;
+        }
+
+        public ApiCallsBuilder AddRange(IEnumerable<ApiCall> calls) {
+            _calls.AddRange(calls ?? throw new ArgumentNullException(nameof(calls)));
+            return this;
+        }
+
+        /// <summary>Adds the same <paramref name="prototype"/> call <paramref name="times"/> times.</summary>
+        public ApiCallsBuilder Repeat(ApiCall prototype, int times) {
+            if (times < 1) throw new ArgumentOutOfRangeException(nameof(times));
+            for (var i = 0; i < times; i++)
+                Add(prototype);
+            return this;
+        }
+
+        /// <summary>Run with the specified degree of parallelism (≥ 1).</summary>
+        public ApiCallsBuilder UseParallel(int maxDegree) => SetDegree(Math.Max(1, maxDegree));
+
+        /// <summary>Run sequentially (same as <c>UseParallel(1)</c>).</summary>
+        public ApiCallsBuilder UseSequential() => SetDegree(1);
+
+        private ApiCallsBuilder SetDegree(int degree) {
+            _parallelism = degree;
+            return this;
+        }
+
+        public ApiCalls Build() {
+            if (_calls.Count == 0)
+                throw new InvalidOperationException("At least one ApiCall is required.");
+            return new ApiCalls(_calls, _parallelism);
+        }
+    }
+}