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/NumberedStateChanger.cs

@@ -0,0 +1,22 @@
+using Beam.Abstractions;
+
+namespace Beam.Dynamic {
+    public class NumberedStateChanger(NumberedStateChanger.MoveState moveState) : IStateChangeBehaviour {
+        public delegate void MoveState(IState state, int amount);
+        public MoveState MoveStateDlgte { get; set; } = moveState;
+
+        public virtual void Apply(IState state, object stimulus) {
+            if (stimulus is not int amount)
+                throw new ArgumentException(string.Format(Exceptions.Exceptions.num_state_changer_stimulus_must_be_int, stimulus.GetType().Name), nameof(stimulus));
+            Apply(state, amount);
+        }
+
+        public virtual void Apply(IState state, int amount) {
+            MoveStateDlgte(state, amount);
+        }
+
+        public NumberedStateChanger(IStateChangeBehaviour behavior) : this((x, i) => {
+            behavior.Apply(x, i);
+        }) {}
+    }
+}