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.

build.rs

@@ -151,12 +151,13 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
             .define("LIBCZI_BUILD_AZURESDK_BASED_STREAM", "OFF")
             .define("LIBCZI_BUILD_PREFER_EXTERNALPACKAGE_RAPIDJSON", "OFF")
             .define("LIBCZI_BUILD_LIBCZIAPI", "ON")
-            // vendored Eigen 3.4.0 headers (eigen/, see its README) so neither a
-            // network clone (libCZI's gitlab ExternalProject) nor a system
-            // libeigen3-dev package is required; Eigen3_DIR finds the package
-            // via eigen/Eigen3Config.cmake
+            // Eigen 3.4.0 comes from the shallow git submodule (eigen/, pinned
+            // at the official tag 3.4.0) so neither a network clone (libCZI's
+            // gitlab ExternalProject) nor a system libeigen3-dev package is
+            // required; Eigen3_DIR finds the small interface config in
+            // eigen-cfg/Eigen3Config.cmake
             .define("LIBCZI_BUILD_PREFER_EXTERNALPACKAGE_EIGEN3", "ON")
-            .define("Eigen3_DIR", manifest_dir.join("eigen"));
+            .define("Eigen3_DIR", manifest_dir.join("eigen-cfg"));
 
         if is_cross {
             if is_windows {