feat: add Empty method and fix link handling

Added a static `Empty` method to `DownloadEnumerable` for
creating an empty instance. Updated link handling in
`SequentialDownloader` to use `AbsoluteUri` instead of
`ToString()`, ensuring correct link representation.

These changes improve usability and ensure consistency in
link formatting.

Beam/SourceLinkBuilder.cs

@@ -139,7 +139,7 @@ namespace Beam {
         /// Produces a deep copy whose <see cref="Segments"/> and contained collections are detached from the original.
         /// </summary>
         public SourceLinkBuilder Clone()
-            => new SourceLinkBuilder(Protocol, Host) {
+            => new SourceLinkBuilder(Host, Protocol) {
                 Segments = [.. Segments.Select(static x => x.Clone())]
             };