fix(build): use pre-built Eigen3 from deps/ instead of bundled 3.3.7

deps_src/eigen contained Eigen 3.3.7 from the ImageMap fork, which
predates the Eigen::all placeholder (requires Eigen 3.4+). libigl in
deps_src uses Eigen::all, causing build failures. Switch to find_package
so the Eigen3::Eigen target resolves to the Eigen 5.0.1 installed by
deps/ CMakeLists via CMAKE_PREFIX_PATH.

deps_src/eigen/CMakeLists.txt

@@ -1,15 +1,13 @@
 cmake_minimum_required(VERSION 3.13)
 project(eigen)
 
-add_library(eigen INTERFACE)
+# Use the pre-built Eigen from deps/ (Eigen3Config.cmake is installed there via CMAKE_PREFIX_PATH).
+# The bundled headers in this directory are an older version and must not be used directly,
+# as libigl and admesh require Eigen3::Eigen with Eigen 3.4+ features (e.g. Eigen::all).
+find_package(Eigen3 REQUIRED NO_MODULE)
 
-target_include_directories(eigen SYSTEM
-    INTERFACE
-        ${CMAKE_CURRENT_SOURCE_DIR}
-)
-
-# Eigen is header-only, so we only need to specify the include directory
-# The headers are in the Eigen/ subdirectory structure
-
-# Alias so that targets linking Eigen3::Eigen also work
-add_library(Eigen3::Eigen ALIAS eigen)
+# Provide a plain "eigen" alias for any code that links to the unnamespaced target.
+if(NOT TARGET eigen)
+    add_library(eigen INTERFACE)
+    target_link_libraries(eigen INTERFACE Eigen3::Eigen)
+endif()