fix: add missing self._lock to KnowledgeGraph.close()

TDD: test first, failed, fixed, passed.

Igor fixed query_relationship/timeline/stats in an earlier commit.
close() was the last method touching self._connection without
holding the lock.

Closes #883.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/test_kg_thread_safety.py

@@ -0,0 +1,13 @@
+"""TDD: KnowledgeGraph.close() must hold self._lock."""
+
+import inspect
+from mempalace.knowledge_graph import KnowledgeGraph
+
+
+class TestKGCloseLock:
+    def test_close_holds_lock(self):
+        src = inspect.getsource(KnowledgeGraph.close)
+        assert "self._lock" in src, (
+            "close() does not acquire self._lock. "
+            "Closing while a read/write is in progress can corrupt data."
+        )