Refactor fluent download pipelines

Beam.Fluent/TransformStage.cs

@@ -0,0 +1,12 @@
+using Beam.Data;
+using Beam.Downloaders;
+using Beam.Dynamic;
+using Beam.Models;
+
+namespace Beam.Fluent;
+
+internal sealed class TransformStage<RawType, OutType>(DownloadContextBuilder<RawType> CtxBuilder) : ITransformStage<RawType, OutType> {
+    public IContextStage<RawType, OutType> WithTransformer(AsyncTransformer<RawType, OutType> transformer) {
+        return new ContextStage<RawType, OutType>(CtxBuilder, transformer);
+    }
+}