COMP: Fix FFTWF-only build on release-5.4

[hjmjohnson]

Fixes the FFTWF-only (single-precision FFTW) build on release-5.4. Stacked on release-5.4. Companion to #6330 (same fixes on main).

The two defects (masked by the default dual-precision build)

1. CMake/itkExternal_FFTW.cmake set FFTW_INCLUDE from the double-precision FFTW3_INCLUDE_DIRS inside the ITK_USE_FFTWF block, leaving it empty in single-precision-only builds (fftw3.h not found).
2. The FFTW factory instantiated both precisions. FFTImageFilterFactory<FFTWxxx> now consults per-precision predicates (FFTImageFilterEnableFloat/Double) guarded by if constexpr. Each FFTW filter header specializes the predicate to false_type for the absent precision, so the factory does not instantiate fftw::ComplexToComplexProxy<double> (undefined when ITK_USE_FFTWD=OFF).

The specializations live in each FFTW filter header (next to FFTImageFilterTraits<FFTWxxx>), not in a .cxx, so every translation unit that instantiates the factory — including itkTestDriverIncludeRequiredFactories.cxx — sees them. (This is the fix learned from #6330, where the .cxx-only specializations left other TUs broken.)

Local verification

Built ITKTestKernel on macOS arm64 with ITK_USE_FFTWF=ON / ITK_USE_FFTWD=OFF / ITK_USE_SYSTEM_FFTW=OFF against release-5.4: staged FFTW built its own fftw3.h, and itkTestDriverIncludeRequiredFactories.cxx compiles and links.

[greptile-apps]

Greptile Summary

This PR fixes two bugs that caused build failures in FFTWF-only (single-precision) configurations on release-5.4, and temporarily adds a CI-validation commit to force the Pixi build matrix to exercise that path.

• CMake fix: itkExternal_FFTW.cmake now correctly uses FFTW3f_INCLUDE_DIRS (not FFTW3_INCLUDE_DIRS) inside the ITK_USE_FFTWF block, so staged FFTW installs its fftw3.h on the include path in single-precision-only builds.
• Factory fix: FFTImageFilterFactory gains FFTImageFilterEnableFloat/FFTImageFilterEnableDouble compile-time predicates guarded by if constexpr; each FFTW filter header specializes the absent precision to std::false_type, preventing instantiation of undefined fftw::ComplexToComplexProxy<double> when ITK_USE_FFTWD=OFF.
• CurvatureRegistrationFilter: Guard narrowed from ITK_USE_FFTWF || ITK_USE_FFTWD to ITK_USE_FFTWD only, reflecting that its implementation uses fftw_plan (double-precision) exclusively; tests fall back from double to float pixel type under FFTWF-only builds.

Confidence Score: 4/5

The core C++ and CMake fixes are correct and safe to merge; the pyproject.toml change must be dropped first.

The CMake one-liner fix, the factory predicate mechanism, and the CurvatureRegistrationFilter guard narrowing are all sound. The only blocker is the pyproject.toml commit that hardcodes FFTWF-only flags into the default configure task — the author explicitly flags it as temporary and marks it for removal before the PR is marked ready. Merging without removing that commit would permanently alter the default Pixi developer build configuration for all contributors.

pyproject.toml — the WIP FFTW flags must be reverted before merging.

Important Files Changed

Filename	Overview
CMake/itkExternal_FFTW.cmake	Fixes the root CMake bug: FFTW3_INCLUDE_DIRS (double-precision) was incorrectly set for FFTW_INCLUDE inside the ITK_USE_FFTWF block; corrected to FFTW3f_INCLUDE_DIRS.
Modules/Filtering/FFT/include/itkFFTImageFilterFactory.h	Adds FFTImageFilterEnableFloat/Double predicates (defaulting to true_type) and guards each registration block with if constexpr, preventing instantiation of undefined FFTW proxies when a precision is absent.
Modules/Filtering/FFT/include/itkFFTWComplexToComplexFFTImageFilter.h	Adds FFTImageFilterEnableFloat/Double specializations to false_type guarded by preprocessor, and a direct fftw3.h include. Has one spurious extra blank line before namespace close.
Modules/Registration/PDEDeformable/include/itkCurvatureRegistrationFilter.h	Correctly restricts the filter guard to ITK_USE_FFTWD only; removes the dead branch that declared RealTypeDFT=double while linking against the float library.
pyproject.toml	Temporary WIP commit hardcoding FFTWF-only flags into the default configure task. PR author explicitly marks this as a drop-before-merge commit; merging as-is would permanently change the default Pixi build to single-precision-only.
Modules/Filtering/FFT/include/itkFFTWForwardFFTImageFilter.h	Adds per-precision enable specializations and direct fftw3.h include, consistent with the pattern applied across all FFTW filter headers.
Modules/Filtering/FFT/test/itkFFT1DImageFilterTest.cxx	Falls back from double to float pixel type when ITK_USE_FFTWD is not defined, allowing the test to run under FFTWF-only builds.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[FFTImageFilterFactory constructor] --> B{FFTImageFilterEnableFloat::value?}
    B -->|true constexpr| C[Register float precision]
    B -->|false constexpr| D[Skip — no instantiation]
    A --> E{FFTImageFilterEnableDouble::value?}
    E -->|true constexpr| F[Register double precision]
    E -->|false constexpr| G[Skip — no instantiation]
    H[FFTW filter header] --> I{ITK_USE_FFTWF defined?}
    I -->|NO| J[EnableFloat specializes to false_type]
    I -->|YES| K[Inherits true_type default]
    H --> L{ITK_USE_FFTWD defined?}
    L -->|NO| M[EnableDouble specializes to false_type]
    L -->|YES| N[Inherits true_type default]

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_{Reviews (2): Last reviewed commit: "COMP: Restrict CurvatureRegistrationFilt..." | Re-trigger Greptile}

[hjmjohnson]

Companion CI-validation PR #6330 (WIP, targets main) exercises the FFTWF-only build path that this fix addresses — it forces ITK_USE_FFTWF=ON / ITK_USE_FFTWD=OFF on the Pixi-Cxx leg, which the default dual-precision matrix never covers.

[hjmjohnson]

@dzenanz Build a 3rd-party app against this, identified an issue with placement of code in .cxx that needs to be in .h files.

Adding more CI testing (temporarily) to investigate the completeness of the solution.

[hjmjohnson]

Dropped the WIP CI-forcing commit (f070dd9c77). The only resulting change vs. the approved revision is pyproject.toml reverting to the base configure task — the 5 fix commits are byte-identical. The FFTWF-only build is exercised by companion PR #6330.

[hjmjohnson]

/azp run

[hjmjohnson]

/azp run ITK.macOS.Python