fix(run-journal): drop obsolete writer lock state

api/run_journal.py

@@ -451,8 +451,6 @@ class RunJournalWriter:
         self.session_id = _validate_id(session_id, "session_id")
         self.run_id = _validate_id(run_id, "run_id")
         self.session_dir = Path(session_dir) if session_dir is not None else None
-        self._path = _run_path(self.session_id, self.run_id, session_dir=self.session_dir)
-        self._lock = _lock_for(self._path)
 
     def append_sse_event(self, event_name: str, payload=None) -> dict | None:
         # Live-UI-only telemetry (metering) has no recovery value in the journal:

tests/test_run_journal_writer_ordering.py

@@ -72,3 +72,10 @@ def test_metering_skip_still_does_not_reserve_or_write(tmp_path):
     assert writer.append_sse_event("metering", {"tps": 10}) is None
     assert not (tmp_path / "_run_journal" / "meter_session" / "meter_run.jsonl").exists()
     assert writer.append_sse_event("token", {"text": "kept"})["seq"] == 1
+
+
+def test_unused_writer_does_not_allocate_a_registry_lock(tmp_path):
+    parent = str(tmp_path / run_journal.RUN_JOURNAL_DIR_NAME / "unused_session")
+    writer = run_journal.RunJournalWriter("unused_session", "unused_run", session_dir=tmp_path)
+    assert writer.append_sse_event("metering", {"tps": 10}) is None
+    assert not any(key[0] == parent for key in run_journal._WRITER_LOCKS)