The eigen/ directory was a byte-identical copy of upstream libeigen/eigen
tag 3.4.0 (commit 3147391d, the same release libCZI pins in
cmake/ExternalEIGEN3.cmake). Replace the copy with a shallow git submodule
(.gitmodules: shallow = true) for verifiable provenance and smaller fork.
The Eigen3::Eigen interface config moves to eigen-cfg/Eigen3Config.cmake
(the Eigen repo only ships the Eigen3Config.cmake.in template), and
build.rs points Eigen3_DIR there.
w.pomp