Introduce vm.state.type_mutex to serialize type mutation and version-tag assignment, matching CPython's free-threading model.

• Add with_type_lock() helper and split modified/assign_version_tag into inner (lock-free) and public (lock-acquiring) variants
• Add version_for_specialization() and lookup_ref_and_version_interned() for atomic type-cache lookups under the lock
• Wrap all type-mutating paths (bases, annotations, module, type_params, doc, SetAttr) with the type lock
• Drop old values outside the lock to prevent deadlock from weakref callbacks that may re-enter specialization
• Fix _ctypes set_attr calls to route through proper SetAttr path
• Reinitialize type_mutex after fork