Merge branch 'feature/core-lifecycle' into main

Reconcile close reason tracking: feature branch's MarkClosed() and
ShouldSkipFlush/FlushAndCloseAsync now use main's ClientClosedReason
enum. ClosedState enum retained for forward compatibility.

.slopwatch/config.json.example

@@ -0,0 +1,10 @@
+{
+  "suppressions": [
+    {
+      "ruleId": "SW002",
+      "pattern": "**/Generated/**",
+      "justification": "Generated code from protobuf/gRPC compiler - cannot be modified"
+    }
+  ],
+  "globalSuppressions": []
+}