COMP: Pin vendored Eigen3 fork to upstream tagged release 5.0.1

Repoint Modules/ThirdParty/Eigen3/UpdateFromUpstream.sh from the
moving master-snapshot tag

    for/itk-20260509-599d71ab-p2-prose   (libeigen/master HEAD)

to the SemVer-stable release tag

    for/itk-eigen-5.0.1                  (libeigen 5.0.1 + 6 ITK overlays)

The fork-side tag for/itk-eigen-5.0.1 is built by branching off
libeigen/eigen 5.0.1 (bc3b39870) and cherry-picking the same 6 ITK
overlay patches that the master-snapshot fork tag carried:

  - .gitattributes: relax content checks for lapacke.h
  - CMakeLists.txt: stub for vendored INTERFACE-only use
  - README.kitware.md: fork purpose + local-patches documentation
  - CMakeLists.txt: register eigen_internal in ITK targets +
    parametrize Eigen3Config.cmake.in
  - CMakeLists.txt: post-import greptile fixups
  - CMakeLists.txt: trim eigen_internal install comments to fit
    ITK prose budget

Pinning to a release tag (rather than a master snapshot) gives ITK
stable, reproducible upstream versioning and avoids inheriting any
post-5.0.1 master-branch development changes.

Author: hjmjohnson

Modules/ThirdParty/Eigen3/UpdateFromUpstream.sh

@@ -13,7 +13,7 @@ readonly ownership="Eigen Upstream <kwrobot@kitware.com>"
 exact_tree_match=false
 readonly subtree="Modules/ThirdParty/Eigen3/src/itkeigen"
 readonly repo="https://github.com/InsightSoftwareConsortium/eigen"
-readonly tag="for/itk-20260501-879885e1"
+readonly tag="for/itk-eigen-5.0.1"
 readonly paths="
 Eigen/AccelerateSupport
 Eigen/Cholesky