ENH: Modernize image iterators to eliminate const_cast via VIsConst templating

Replace ITK's parallel `XxxConstIterator` / `XxxIterator` class pairs with a single `XxxIteratorBase` class template, eliminating the `const_cast` escape hatches that the legacy non-const leaves rely on.

Motivation

The legacy iterator family is structured as twin classes — `ImageRegionConstIterator` and `ImageRegionIterator` — where the writable leaf inherits from the const base and uses `const_cast` to recover write access:

```cpp
void Set(const PixelType & value) const {
this->m_PixelAccessorFunctor.Set(
*(const_cast(this->m_Buffer + this->m_Offset)),
value);
}
```

This pattern repeats across the iterator family. It works, but:

- Cast-removed write access is invisible at the type system level — any future bug that lets a `ConstIterator` reach the cast site silently mints a writable pointer to a const buffer.
- Maintaining two parallel headers per iterator (header + .hxx, const + non-const = 4 files) doubles the surface for drift; bug fixes regularly land in only one half of the pair (see commit history for `m_PixelAccessor` vs. `m_PixelAccessorFunctor`).
- Conversion between non-const and const iterators is by inheritance + slicing, which interferes with reverse-iterator construction patterns.

The fix is to collapse each pair into a single class template parameterized on a compile-time `bool VIsConst` flag. Pointer-like members switch element-const via `std::conditional_t`. Write methods (`Set`, non-const `Value`) are SFINAE-gated on `!VIsConst`. The legacy class names are preserved as alias templates so all consumers compile unchanged.

Status: branch modernize-iterators-remove-const-cast

Local branch on `hjmjohnson/ITK` carries 13 ENH commits covering the full image-iterator family:

| Unit | Cluster |
|------|---------|
| 1 | `ImageConstIterator` / `ImageIterator` (root) |
| 2 (×7 pairs) | `ImageIteratorWithIndex` family: Region / Linear / Slice / Random / RandomNonRepeating / RegionExclusion |
| 3 | `ImageScanlineIterator` |
| 4 | `ImageReverseIterator` / `ImageRegionReverseIterator` |
| 5 | `LineIterator` |
| 6 | `ConditionalIterator` + `FloodFilled*` cluster |
| 7 | `ImageRegionIterator` (non-WithIndex) |

Each unit is one self-contained ENH commit. Build is green at 1360/1360 targets; 3270/3270 ctests passed at the last full validation.

Plus the design spike `ENH: Add NeighborhoodIteratorBase spike with runtime tests` which prototypes the same templated base for the neighborhood family but does not yet wire it into the legacy class names.

Out of scope (follow-up PRs)

- **Neighborhood iterators** (`ConstNeighborhoodIterator`, `NeighborhoodIterator`, `ConstShapedNeighborhoodIterator`, `ShapedNeighborhoodIterator`). Four `const_cast` sites remain. Removing them requires more than a templating pass: the legacy `ConstShapedNeighborhoodIterator` *privately inherits from* the writable `NeighborhoodIterator`, so the constructor cannot accept its declared `const ImageType *` without casting. The spike replaces this with composition. That replacement is roughly 1700 lines and warrants its own focused review cycle.
- **Comment-only follow-up**: a small set of legacy docblocks would benefit from rewording, but doing so in this PR would inflate the diff and obscure the structural change. Tracked separately in the working branch.