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.

eigen-cfg/Eigen3Config.cmake

@@ -0,0 +1,24 @@
+# SPDX-License-Identifier: MPL-2.0
+#
+# CMake package interface for the Eigen submodule (eigen/), pinned at the
+# official tag 3.4.0 (commit
+# 3147391d946bb4b6c68edd901f2add6ac1f31f8c - the exact release libCZI pins in
+# its cmake/ExternalEIGEN3.cmake). The Eigen release tree only ships the
+# template cmake/Eigen3Config.cmake.in, which is expanded by Eigen's own CMake
+# build (deliberately skipped by libCZI), so this small config provides the
+# Eigen3::Eigen interface target used via
+#   find_package(Eigen3 REQUIRED NO_MODULE)
+# with LIBCZI_BUILD_PREFER_EXTERNALPACKAGE_EIGEN3=ON.
+#
+# Eigen is header-only (MPL-2.0; see COPYING.MPL2 in the submodule) and the
+# submodule is checked out shallow (see .gitmodules), so libczirw-sys builds
+# need neither network access during the build (libCZI's default Eigen
+# ExternalProject clone from gitlab.com is flaky in CI), a full eigen git
+# history, nor a system libeigen3-dev package.
+if(NOT TARGET Eigen3::Eigen)
+  get_filename_component(_eigen3_root "${CMAKE_CURRENT_LIST_DIR}/../eigen" ABSOLUTE)
+  add_library(Eigen3::Eigen INTERFACE IMPORTED GLOBAL)
+  set_target_properties(Eigen3::Eigen PROPERTIES
+    INTERFACE_INCLUDE_DIRECTORIES "${_eigen3_root}")
+  set(EIGEN3_INCLUDE_DIR "${_eigen3_root}")
+endif()