[hjmjohnson]

Closes #6141 and fixes #7003: every image iterator now takes its access from the constness of the image type, so XxxIterator<const ImageType> has no Set() and no writable Value(), and no iterator in the family needs const_cast. Supersedes #7021, whose commit is the first commit here, authored by @mymonin and kept unchanged.

Merge-order note: this conflicts with #7041 in itkImageRandom*IteratorWithIndex (#7041's detail::RandomIndexInRegion moves with the class into itkImageRandomIteratorWithIndex.h); whichever lands second needs that one-hunk rebase. Two bugs that this PR exposes but that also exist on main are fixed separately: #7068 (RandomPermutation assignment loses the shuffled order) and #7069 (reverse region iterator span is one pixel off after SetIndex() or conversion). This PR takes both fixes on rebase.

Design

• The writable name carries the implementation: ImageRegionIterator<TImage> derives from ImageIterator<TImage>, and pixel pointers are const exactly when TImage is const, so writes need no cast.
• Set() and non-const Value() are removed for const TImage: SFINAE in C++17, a requires clause when concepts are available, which gives a "constraints not satisfied" diagnostic.
• ImageRegionConstIterator<T> is a thin class deriving from ImageRegionIterator<const T>, with deduction guides for SmartPointer, ConstPointer, and raw pointers. Each header provides both names, and the Const headers and .hxx files remain as one-line includes.
• detail::IsImageIteratorConvertible_v is the single statement of allowed conversions: same image type, never dropping const.
• C++20 compilers get operator== plus operator<=> instead of six comparison operators; C++26 compilers get = delete("…") naming the forbidden read-only to writable conversion. Both are behind feature-test macros (__cpp_concepts, __cpp_impl_three_way_comparison, __cpp_deleted_function).
• The flood-filled iterators keep the writable image pointer their constructor receives instead of inverting the ConditionalConstIterator hierarchy.

API changes and their measured cost

Removed, each by design:

• Writing through any iterator over a const image (const-qualified images can be modified via itk::ImageRegionIterator<const ImageType> #7003), including ImageReverseConstIterator::Set(), which was public.
• Converting a read-only iterator into a writable one (the protected XxxIterator(const XxxConstIterator &) constructors and operator=).
• The XxxIterator<T> to XxxConstIterator<T> base-class relation: XxxIterator<T>::Superclass is now the parent iterator, and a function taking a non-const XxxConstIterator<T> & no longer accepts an XxxIterator<T>. Conversion to a read-only iterator is by constructor.

Other changes:

• ImageReverseIterator, ImageRegionReverseIterator, and their Const forms now share one class; ImageReverseConstIterator gains operator++.
• PathIterator::Value() referred to a nonexistent member and could not compile; it now works.
• Converting a forward iterator to a region reverse iterator now sets the row span from the current position (the end offset was computed from an unset begin offset). The one-pixel offset in that span is fixed separately in BUG: Keep reverse region iterators inside the region after SetIndex #7069.
• Kept from ITK 5: ImageIteratorDimension on the indexed iterators, and construction of the exclusion and frequency iterators from a plain ImageIteratorWithIndex.

Blast radius, measured in the downstream build forest (ITK with Python wrapping and this branch, merged locally with #7041 and #7056): ANTs builds and passes 179/179, BRAINSTools builds and passes 458/458, and SimpleITK, RTK, Cleaver, Impact, TractographyTRX, and PerformanceBenchmarking build. The failures, in elastix (vnl_determinant), c3d (removed VNL and orientation APIs), SimpleITKFilters (duplicate target), TubeTK (missing include and members), and several remote modules (deprecated ITK_DISALLOW_COPY_AND_ASSIGN), are identical with main's headers. No downstream code depended on the removed base-class relation or the writable conversions.

Testing

• itkImageIteratorsGTest: compile-time checks that every writable iterator has Set() for Image<int> and not for const Image<int>, that the Const names have none, and conversion and comparison rules, Value() assignable only for a non-const image, ImageIteratorDimension on the indexed iterators, and construction from ImageIteratorWithIndex.
• itkFrequencyIteratorsGTest: the four frequency iterators accept any ImageIteratorWithIndex over the same image type.
• All iterator families compiled with -Wall -Wextra under -std=c++17, c++20, c++23, and c++26 with Apple clang 21 and Homebrew clang 23 (exercising both sides of every #if); no GCC was available locally, so GCC's C++20 path is first exercised by CI.
• Local full ITK build (macOS arm64, conda clang, Release): 0 new warnings, 6508/6508 tests pass; each of the original 10 commits built on its own. After the review fixes, the full build is clean and the ITKCommon and ITKImageFrequency suites pass 738/738
• pre-commit run --all-files passes.

AI assistance (Claude Code) carried out the conversion with scripts kept for review, ran the probes, and ran the downstream forest; the author reviewed the changes.

[hjmjohnson]

@greptileai please review the latest commits (6dca3f7).

[hjmjohnson]

@greptileai please review the latest commits (1e416e2).