Reorganize tests by API and service concerns

README.md

@@ -7,6 +7,12 @@ Fresh full-stack starter scaffold:
 - `RpgRoller.Tests/`: xUnit integration-heavy test project
 - `RpgRoller.sln`: solution used by local CI script
 
+Test layout:
+
+- `RpgRoller.Tests/Api/`: API integration tests grouped by feature concern
+- `RpgRoller.Tests/Services/`: service-level tests grouped by domain concern
+- `RpgRoller.Tests/Support/`: shared test harnesses/builders/helpers
+
 ## Code Organization
 
 Backend: