gh-157217: Add NEWS blurb for mappingproxy dict_merge fix

Author: KingLizard1020

Misc/NEWS.d/next/Core_and_Builtins/2026-09-10-16-20-00.gh-issue-157217.dirlock.rst

@@ -0,0 +1,4 @@
+Fix :func:`dir` raising :exc:`RuntimeError` on the free-threaded build
+when another thread concurrently inserts into a class ``__dict__``.
+``dict.update()`` and related merges now take the locked dict-to-dict path
+when the source is a :class:`types.MappingProxyType` wrapping a dict.