Tests: xUnit project with HAR fixtures (50 tests); fix relative delivery/HLS URLs resolving to file:// on Linux

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

README.md

@@ -92,6 +92,9 @@ Without a cookie you can still browse creators listed under
 dotnet test            # unit tests (models, HLS rewriter, API client with fixtures from the HAR)
 ```
 
+The test host needs the ASP.NET Core 8 shared runtime (the plugin references
+Jellyfin's controllers): on Arch `sudo pacman -S aspnet-runtime-8.0`.
+
 Local end-to-end test with a disposable Jellyfin matching the Pi5 version:
 
 ```bash