refactor: replace vendored Eigen copy with shallow git submodule pinned to tag 3.4.0

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.

.gitmodules

@@ -1,3 +1,7 @@
 [submodule "libczi"]
 	path = libczi
 	url = https://github.com/ZEISS/libczi.git
+[submodule "eigen"]
+	path = eigen
+	url = https://gitlab.com/libeigen/eigen.git
+	shallow = true