fix: add missing thread_barrier to mthreads spec _core.py

Upstream triton.experimental.gluon.language.__init__.py imports thread_barrier
from ._core, but the mthreads spec_path copy only had the older barrier()
function. This caused 10 test collection errors across test_frontend and tle
tests.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: Stardep-lmc

third_party/mthreads/backend/spec/triton/experimental/gluon/language/_core.py

@@ -573,6 +573,14 @@ def num_ctas(_semantic=None):
     return _semantic.num_ctas()
 
 
+@builtin
+def thread_barrier(_semantic=None):
+    """
+    Insert a barrier to synchronize threads within a CTA.
+    """
+    return _semantic.debug_barrier()
+
+
 @builtin
 def barrier(*, cluster: bool = False, _semantic=None):
     """