Restructure solution layout by module

src/Signals/StellaOps.Signals/Persistence/ICallgraphRepository.cs

@@ -0,0 +1,13 @@
+using System.Threading;
+using System.Threading.Tasks;
+using StellaOps.Signals.Models;
+
+namespace StellaOps.Signals.Persistence;
+
+/// <summary>
+/// Persists normalized callgraphs.
+/// </summary>
+public interface ICallgraphRepository
+{
+    Task<CallgraphDocument> UpsertAsync(CallgraphDocument document, CancellationToken cancellationToken);
+}