The new :class:`site.StartupState` class lets callers batch-process
:pep:`829` startup configuration files across multiple site directories
before any startup code runs, with public
:meth:`~site.StartupState.addsitedir`,
:meth:`~site.StartupState.addusersitepackages`,
:meth:`~site.StartupState.addsitepackages`, and
:meth:`~site.StartupState.process` methods.  The signature of
:func:`site.addsitedir` is unchanged from Python 3.14.  The
:data:`!defer_processing_start_files` argument and the
``process_startup_files()`` function added earlier in the 3.15 cycle have
been removed; use :class:`!site.StartupState` instead.
