feat: add diagnostic source for process lifecycle

The library publishes process lifecycle events through a `DiagnosticListener` named
`Geekeey.Process`. Subscribe through `DiagnosticListener.AllListeners` and handle the
`starting`, `started`, and `finished` events. Each event value is a mutable `Hashtable`
that is shared by the lifecycle, so values added by a listener remain available to later
stages.

src/process.tests/DiagnosticListenerTests.cs

@@ -122,8 +122,7 @@ internal sealed class DiagnosticListenerTests
 			_name = name;
 		}
 
-		public IReadOnlyList<KeyValuePair<string, Hashtable>> Events => _events.AsReadOnly();
-
+		public IReadOnlyList<KeyValuePair<string, Hashtable>> Events => _events;
 		public Action<string, Hashtable>? ContextChanged { get; set; }
 
 		public void OnNext(DiagnosticListener value)