[skip ci] Add st.first comments (generated by ChatGPT)

Author: rwgk

include/pybind11/detail/type_caster_base.h

@@ -576,6 +576,8 @@ handle smart_holder_from_unique_ptr(std::unique_ptr<T, D> &&src,
     if (!src) {
         return none().release();
     }
+    // st.first is the subobject pointer appropriate for tinfo (may differ from src.get()
+    // under MI/VI). Use this for Python identity/registration, but keep ownership on T*.
     void *src_raw_void_ptr = const_cast<void *>(st.first);
     assert(st.second != nullptr);
     const detail::type_info *tinfo = st.second;
@@ -657,6 +659,8 @@ handle smart_holder_from_shared_ptr(const std::shared_ptr<T> &src,
         return none().release();
     }
 
+    // st.first is the subobject pointer appropriate for tinfo (may differ from src.get()
+    // under MI/VI). Use this for Python identity/registration, but keep ownership on T*.
     void *src_raw_void_ptr = const_cast<void *>(st.first);
     assert(st.second != nullptr);
     const detail::type_info *tinfo = st.second;