Improvements and fixes for lists that were broken in the last release.

• Added some tests for the tagged_ptr_union utility
• The tagged_ptr_union utility no longer distinguishes between T and and const T, because it is a narrow wrapper over tricks with std::bit_cast, which also does not; generally the client code using this utility is const correct, and it is less verbose if the utility itself ignores const (i.e., to avoid many overloads)
• entry_ref_codec::create_item_entry_ref can now create an entry_ref_union from a const Item *; this fixes the implicit construction of a list const_iterator from a pointer to an item in the list, an important bit of syntactic sugar that had broken during the big C++23 refactor
• A test was added for the above functionality
• stailq's find_predecessor_if projection was missing its default argument
• Fix the paths in the header file comments
• A few improvements to the documentation