ENH: Retire FEM modules to InsightSoftwareConsortium/ITKFEM remote

[hjmjohnson]

Retires the FEM modules (ITKFEM, ITKFEMRegistration) from the ITK main tree, replacing them with a Modules/Remote/FEM.remote.cmake breadcrumb pointing at the new full-history remote repository InsightSoftwareConsortium/ITKFEM. Phase 1 of #6511.

What this does

• Removes Modules/Numerics/FEM (ITKFEM) and Modules/Registration/FEM (ITKFEMRegistration) — both opt-in (EXCLUDE_FROM_DEFAULT); no in-tree module depended on them.
• Adds Modules/Remote/FEM.remote.cmake (MODULE_COMPLIANCE_LEVEL 1, pinned GIT_TAG) with a loud enable-time warning that the module is unmaintained and slated for removal in ITK 7, asking dependents to comment on Tracking: Retire the FEM module (ITKFEM / ITKFEMRegistration) to reduce maintenance burden #6511.
• Removes the two FEM-based deformable-registration examples + their data, drops FEM references from the Registration module-group doc and the license attribution list, and documents the retirement in the ITK 6 migration guide.

History preservation + verification

The remote repo was produced with git filter-repo from a fresh clone, retaining the complete history (1,290 commits back to the original 2001 check-in by Aljaž Noe, 137 merges) with legacy Code/-era paths normalized onto today's Modules/.../FEM layout. Content is tree-hash identical to upstream/main FEM.

End-to-end verified locally: enabling Module_FEM clones the remote, fires the warning, discovers both nested modules, and builds libITKFEM + libITKFEMRegistration.

[dzenanz]

Mostly looks good.

[greptile-apps]

Greptile Summary

This PR retires the FEM modules from the main ITK tree. The main changes are:

• Removed the in-tree ITKFEM and ITKFEMRegistration module sources, tests, and examples.
• Added a FEM remote-module breadcrumb pinned to the new external repository.
• Updated migration, remote-module, registration-group, and license documentation.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.
• The remote-module path is opt-in and matches the documented migration path.
• Removed example targets no longer have active in-tree build references.

Important Files Changed

Filename	Overview
Modules/Remote/FEM.remote.cmake	Adds the opt-in remote-module breadcrumb, pinned repository/tag, compliance level, and warning text.
Examples/RegistrationITKv4/CMakeLists.txt	Removes the FEM-backed registration example targets from the examples build.
Documentation/docs/migration_guides/itk_6_migration_guide.md	Documents the FEM module retirement and the Module_FEM migration path.
CMake/ITKGroups.cmake	Removes ITKFEM from the Registration group description.
Documentation/docs/scientific_ecosystem/remote_modules.md	Adds FEM to the remote modules table with its external repository and retirement status.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
  A[ITK main tree before PR] --> B[In-tree ITKFEM]
  A --> C[In-tree ITKFEMRegistration]
  A --> D[FEM registration examples]
  E[ITK main tree after PR] --> F[Modules/Remote/FEM.remote.cmake]
  F --> G[Module_FEM opt-in]
  G --> H[Fetch InsightSoftwareConsortium/ITKFEM]
  H --> I[Remote ITKFEM]
  H --> J[Remote ITKFEMRegistration]
  E --> K[Migration guide and remote module docs]

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flowchart TD
  A[ITK main tree before PR] --> B[In-tree ITKFEM]
  A --> C[In-tree ITKFEMRegistration]
  A --> D[FEM registration examples]
  E[ITK main tree after PR] --> F[Modules/Remote/FEM.remote.cmake]
  F --> G[Module_FEM opt-in]
  G --> H[Fetch InsightSoftwareConsortium/ITKFEM]
  H --> I[Remote ITKFEM]
  H --> J[Remote ITKFEMRegistration]
  E --> K[Migration guide and remote module docs]

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