Improve performance of enum_ operators by going back to specific implementation (#5887)

* Improve performance of enum_ operators by going back to specific implementation

test_enum needs a patch because ops are now overloaded and this affects their docstrings.

* outline call_impl to save on code size

This does cause more move constructions, as shown by the needed update to test_copy_move. Up to reviewers whether they want more code size or more moves.

* add function_ref.h to PYBIND11_HEADERS.

* Update test_copy_move tests with C++17 passing values just so we can see mostly-not-red tests

* Remove stray TODO

* fix clang-tidy

* fix clang-tidy again. add function_ref.h to test_files.py

* Add static assertion for function_ref lifetime safety in call_impl

Add a static_assert to document and enforce that function_ref is
trivially copyable, ensuring safe pass-by-value usage. This also
documents the lifetime safety guarantees: function_ref is created
from cap->f which lives in the capture object, and is only used
synchronously within call_impl without being stored beyond its scope.

* Add #undef cleanup for enum operator macros

Undefine all enum operator macros after their last use to prevent
macro pollution and follow the existing code pattern. This matches
the cleanup pattern used for the previous enum operator macros.

* Rename PYBIND11_THROW to PYBIND11_ENUM_OP_THROW_TYPE_ERROR

Rename the macro to be more specific and avoid potential clashes with
public macros. The new name clearly indicates it's scoped to enum
operations and describes its purpose (throwing a type error).

* Clarify comments in function_ref.h

Replace vague comments about 'extensions to <functional>' and 'functions'
with a clearer description that this is a header-only class template
similar to std::function but with non-owning semantics. This makes it
clear that it's template-only and requires no additional library linking.

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Co-authored-by: Ralf W. Grosse-Kunstleve <rgrossekunst@nvidia.com>