Simplify initialization of local region iterators in hxx files

[N-Dekker]

Replaced code like T x; x = T(a, b), T x = T(a, b) and auto x = T(a, b) with T x(a, b), for local region iterators.

Cases found, using Notepad++ with the following regular expression: \w+ =[\r\n]*[ ]+Image\w*Iterator.

[greptile-apps]

Greptile Summary

This PR simplifies local iterator variable declarations across 7 .hxx files by replacing the two-step default-construct-then-assign pattern (and auto x = T(a, b) variants) with direct C++17 CTAD-based initialization (T x(a, b)). All deduction guides for ImageRegionIterator, ImageRegionConstIterator, and ImageRegionConstIteratorWithIndex are in place and cover both SmartPointer inputs (explicit guides) and raw pointer inputs (implicit constructor-derived guides), so all resulting types are identical to the originals.

Confidence Score: 5/5

Safe to merge — all changes are semantically equivalent refactors with no behavioral impact.

Every changed site produces the same concrete iterator type as before: CTAD resolves correctly in all cases (SmartPointer inputs hit the explicit deduction guides; raw pointer inputs hit the implicit constructor guides). The one reordering in itkHessian3DToVesselnessMeasureImageFilter.hxx — moving oit construction to after AllocateOutputs() — is actually an improvement. No logic, data flow, or lifetime changes were introduced. All remaining notes are P2 style observations.

No files require special attention.

Vulnerabilities

No security concerns identified.

Important Files Changed

Filename	Overview
Modules/Filtering/BinaryMathematicalMorphology/include/itkBinaryMorphologyImageFilter.hxx	Simplifies ImageRegionIterator declaration from two-line default-construct-then-assign pattern to a single direct construction using CTAD; tmpSEImage is a SmartPointer so the explicit deduction guide applies.
Modules/Filtering/BinaryMathematicalMorphology/include/itkObjectMorphologyImageFilter.hxx	Replaces auto x = ImageRegionConstIterator(...) / auto y = ImageRegionIterator(...) with typed direct-initialization; semantically equivalent, both paths use CTAD.
Modules/Filtering/ImageFeature/include/itkHessian3DToVesselnessMeasureImageFilter.hxx	Moves oit declaration to after AllocateOutputs() and initializes it inline; this is actually a slight correctness improvement since the iterator is now constructed on already-allocated output data.
Modules/Filtering/ImageFeature/include/itkZeroCrossingImageFilter.hxx	Replaces ImageRegionIterator<TOutputImage> it = ImageRegionIterator(output, face) with direct CTAD initialization; cosmetic and equivalent.
Modules/Registration/PDEDeformable/include/itkCurvatureRegistrationFilter.hxx	Collapses five separate declare-then-assign lines into four direct-initialization declarations; CTAD deduction is correct for all four iterators (SmartPointer inputs trigger explicit guides; raw pointers use implicit constructor guides).
Modules/Segmentation/LevelSets/include/itkExtensionVelocitiesImageFilter.hxx	Removes now-unused LocalLevelSetImageType typedef and the early tempIt declaration, replacing the late assignment with a direct inline construction after m_Marcher->Update(); logic is unchanged.
Modules/Segmentation/Watersheds/include/itkWatershedSegmenter.hxx	Two ImageRegionIterator<BoundaryType::face_t> copy-initialize patterns simplified to direct CTAD construction; face is a SmartPointer so the explicit deduction guide fires correctly.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A["Old pattern:\nT x;\nx = T(a, b);\n— or —\nauto x = T(a, b);"] -->|"CTAD + direct init"| B["New pattern:\nT x(a, b);"]
    B --> C{"First arg type?"}
    C -->|"SmartPointer<TImage>"| D["Explicit deduction guide\nImageRegionIterator<TImage>"]
    C -->|"Raw pointer\n(const TImage*)"| E["Implicit constructor guide\nImageRegionIterator<TImage>"]
    D --> F["Same concrete type\nas before"]
    E --> F

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