Restructure solution layout by module

src/Signals/StellaOps.Signals/Models/CallgraphNode.cs

@@ -0,0 +1,12 @@
+namespace StellaOps.Signals.Models;
+
+/// <summary>
+/// Normalized callgraph node.
+/// </summary>
+public sealed record CallgraphNode(
+    string Id,
+    string Name,
+    string Kind,
+    string? Namespace,
+    string? File,
+    int? Line);