ITK: Trim eigen_internal install-rule comments to fit prose budget

Greptile P1 finding on PR InsightSoftwareConsortium/ITK#6249:
the 8-line comment block at CMakeLists.txt:971-981 and the
3-line block in Eigen3Config.cmake.in:7-9 violate ITK's
prose-budget cap of 1 short line per in-source comment.

Both reduce to a single load-bearing line carrying the issue
reference; the rest of the rationale lives in the commit
message and PR body, per AGENTS.md → prose-budget.md.

Issue:    InsightSoftwareConsortium/ITK#6239
Reviewer: greptile P1 on InsightSoftwareConsortium/ITK#6249

Author: hjmjohnson

CMakeLists.txt

@@ -968,17 +968,10 @@ target_include_directories (eigen_internal SYSTEM INTERFACE
 
 # Export as title case Eigen
 set_target_properties (eigen_internal PROPERTIES EXPORT_NAME Eigen)
-# Register in ITK's main export set so find_package(ITK) resolves
-# eigen_internal as an IMPORTED target loaded from ITKTargets.cmake
-# (issue InsightSoftwareConsortium/ITK#6239: SimpleITK FetchContent +
-# FIND_PACKAGE_ARGS fallback). ${ITK3P_INSTALL_EXPORT_NAME} is set by
-# ITK proper in Modules/ThirdParty/Eigen3/src/CMakeLists.txt; if unset
-# (i.e. building outside ITK), this install becomes a no-op.
+# Issue InsightSoftwareConsortium/ITK#6239: ship eigen_internal in ITKTargets.cmake when building inside ITK.
 if(DEFINED ITK3P_INSTALL_EXPORT_NAME)
   install (TARGETS eigen_internal EXPORT ${ITK3P_INSTALL_EXPORT_NAME})
 endif()
-# Also register in the standalone ITKInternalEigen3 export set used by
-# find_package(ITKInternalEigen3) consumers.
 install (TARGETS eigen_internal EXPORT ITKInternalEigen3Targets)
 
 set(EIGEN3_TARGETS_FILE ITKInternalEigen3Targets.cmake)

cmake/Eigen3Config.cmake.in

@@ -4,8 +4,6 @@
 @PACKAGE_INIT@
 
 if (NOT TARGET Eigen3::Eigen)
-  # @EIGEN3_TARGETS_FILE@ is substituted at install-template time to
-  # either Eigen3Targets.cmake (eigen_external / standalone Eigen3) or
-  # ITKInternalEigen3Targets.cmake (eigen_internal / ITK vendored).
+  # @EIGEN3_TARGETS_FILE@ → Eigen3Targets.cmake (external) or ITKInternalEigen3Targets.cmake (vendored).
   include ("${CMAKE_CURRENT_LIST_DIR}/@EIGEN3_TARGETS_FILE@")
 endif (NOT TARGET Eigen3::Eigen)