* Revert PR #5700 production code change (pybind11/detail/struct_smart_holder.h).
```
git checkout b19489145b2c7a117138632d624809dfb3b380bb~1 include/pybind11/detail/struct_smart_holder.h
```
* Introduce `get_internals().get_memory_guarded_delete()`
* [skip ci] Only pass around `memory::get_guarded_delete` function pointer.
* [skip ci] Change a variable name for internal consistency. Add 3 x NOTE: PYBIND11_INTERNALS_VERSION needs to be bumped if changes are made to this struct.
* Add comment: get_internals().get_memory_guarded_delete does not need with_internals()
* Traverse all DSOs to find memory::guarded_delete with matching RTTI.
* Add nullptr check to dynamic_cast overload.
Suggested by ChatGPT for these reasons:
* Prevents runtime RTTI lookups on nullptr.
* Helps avoid undefined behavior if users pass in nulls from failed casts or optional paths.
* Ensures consistent return value semantics and no accidental access to vtable.
* Improve smart_holder unique_ptr deleter compatibility checks across DSOs:
* Replace RTTI-based detection of std::default_delete<T> with a constexpr check to avoid RTTI reliance
* Add type_info_equal_across_dso_boundaries() fallback using type_info::name() for RTTI equality across macOS DSOs
* Rename related flags and functions for clarity (e.g., builtin → std_default)
* Improves ABI robustness and clarity of ownership checks in smart_holder
* Trivial renaming for internal consistency: builtin_delete → std_default_delete
* Add get_trampoline_self_life_support to detail::type_info (passes local testing).
* Polish previous commit slightly.
* [skip ci] Store memory::get_guarded_delete in `detail::type_info` instead of `detail::internals` (no searching across DSOs required).
* Revert change suggested by ChatGPT. After double-checking, ChatGPT agrees this isn't needed.
* Minor polishing.
Ralf W. Grosse-Kunstleve
365d41a4ba