ENH: Add StructuralSimilarityImageFilter at Modules/Filtering

[hjmjohnson]

Adds Modules/Filtering/StructuralSimilarity/ containing StructuralSimilarityImageFilter (SSIM, Wang et al. 2004). Standalone module under Filtering — does not touch ImageCompare or any other existing module. Enabled in CI via configure-ci. Supersedes #6034 (which was stacked on the now-rejected Modules/Beta/ approach in #6085).

What changed from PR #6034

• Previous: SSIM lived at Modules/Beta/StructuralSimilarity/, stacked on ENH: Add Modules/Beta/ container (infra-only for stacked beta-module ingests) #6085 (Modules/Beta/ container, rejected by reviewers)
• Now: Standalone module at Modules/Filtering/StructuralSimilarity/, based directly on upstream/main. No dependency on ENH: Add Modules/Beta/ container (infra-only for stacked beta-module ingests) #6085 or WIP: ENH: Ingest 10 beta modules inline under Modules/Beta/ (stacked on #6085) #6086.
• Module contents unchanged: same filter implementation, same GoogleTest suite (30 test cases), same Python wrapping
• Added -DModule_StructuralSimilarity:BOOL=ON to configure-ci in pyproject.toml

Module layout

Modules/Filtering/StructuralSimilarity/
├── CMakeLists.txt
├── itk-module.cmake          # DEPENDS ITKImageIntensity
│                              # COMPILE_DEPENDS ITKImageFilterBase, ITKSmoothing
│                              # TEST_DEPENDS ITKTestKernel, ITKGoogleTest
├── include/
│   ├── itkStructuralSimilarityImageFilter.h
│   └── itkStructuralSimilarityImageFilter.hxx
├── test/
│   ├── CMakeLists.txt
│   └── itkStructuralSimilarityImageFilterGTest.cxx   # 30 GTest cases
└── wrapping/
    ├── CMakeLists.txt
    └── itkStructuralSimilarityImageFilter.wrap

Local verification

• Configure: clean
• Build: no errors, no warnings
• Tests: 31/32 pass (1 skip: KWStyleTest — not installed locally, passes in CI)
• Pre-commit: all hooks pass

Reviewer feedback addressed (from #6034)

• @blowekamp: VerifyPreconditions override (moved all param/input checks there)
• @blowekamp: Mini-pipeline reconnection using CompositeFilterExample pattern
• @blowekamp: Standalone module — does not perturb ImageCompare's dependency surface
• @thewtex: No Beta staging — direct integration into Filtering/

AI disclosure

Filter implementation and test suite developed with Claude Code assistance across multiple iterations. Relocated from Modules/Beta/ to Modules/Filtering/ per reviewer consensus.