[BUG]: does not work with Eigen 5

### Required prerequisites - [x] Make sure you've read the [documentation](https://pybind11.readthedocs.io). Your issue may be addressed there. - [x] Search the [issue tracker](https://github.com/pybind/pybind11/issues) and [Discussions](https:/pybind/pybind11/discussions) to verify that this hasn't already been reported. +1 or comment there if it has. - [x] Consider asking first in the [Gitter chat room](https://gitter.im/pybind/Lobby) or in a [Discussion](https:/pybind/pybind11/discussions/new). ### What version (or hash if on master) of pybind11 are you using? 98e50b702d9ab784ec918399aaa20adcfd1af9a9 ### Problem description Eigen 5 was released on 30th Sep 2025 <https://gitlab.com/libeigen/eigen/-/releases/5.0.0>. The following macros are no longer available: https://github.com/pybind/pybind11/blob/98e50b702d9ab784ec918399aaa20adcfd1af9a9/tools/FindEigen3.cmake#L36-L44 namely version detection fails. ### Reproducible example code ```text $ cmake -S . -B build … -- Eigen3 version .. found in /usr/include/eigen3, but at least version 3.2.7 is required -- Building tests WITHOUT Eigen, use -DDOWNLOAD_EIGEN=ON to download … ``` ### Is this a regression? Put the last known working version here if it is. Not a regression