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.Dynamic/CommonStateChangers.cs

@@ -0,0 +1,39 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using Beam.Abstractions;
+
+namespace Beam.Dynamic {
+    public static class CommonStateChangers {
+        public static IStateChangeBehaviour LastAsNumber => new NumberedStateChanger((x, i) => {
+            object last = x.GetState()[^1];
+            if (!int.TryParse(last.ToString(), out var number))
+                throw new InvalidOperationException(Exceptions.Exceptions.state_change_error); // TODO use more specific exception
+            x.GetState()[^1] = (number + i).ToString();
+        });
+
+        public static IStateChangeBehaviour Constant => new ConstantStateChanger();
+
+        public static IStateChangeBehaviour NthAsNumber(Index n, bool keepSuffix = true)
+            => new NumberedStateChanger((x, i) => {
+                string? nth = x.GetState()[n]?.ToString();
+                string[] xState = x.GetState();
+                if (nth is null)
+                    throw new InvalidOperationException(Exceptions.Exceptions.state_change_error); // TODO use more specific exception
+                if (!int.TryParse(nth, out var number))
+                    if (keepSuffix) {
+                        string[] split = nth.Split('.');
+                        if (!int.TryParse(split[0], out number))
+                            throw new InvalidOperationException(Exceptions.Exceptions.state_change_error); // TODO use more specific exception
+                        xState[n] = (number + i) + split[1..].Aggregate((x, y) => $"{x}.{y}");
+                        return;
+                    } else
+                        throw new InvalidOperationException(Exceptions.Exceptions.state_change_error); // TODO use more specific exception
+                xState[n] = (number + i).ToString();
+            });
+
+
+    }
+}