ENH: Restore the static LevelSetsv4 examples without VTK rendering

[hjmjohnson]

Restore five LevelSetsv4 examples excluded since ITK 5 by writing their output
instead of rendering through VTK classes ITK no longer has. They move to
Segmentation/LevelSetsv4.

Why these examples, and why they move

These are the only examples of the LevelSetsv4 framework — LevelSetDenseImage,
the Whitaker, Shi, and Malcolm sparse level sets, and
BinaryImageToLevelSetImageAdaptor appear in no other example. They were built
only under if(ITK_VERSION_MAJOR LESS 5) because their visualization classes
moved out of ITK in 2019 (LevelSetsv4Visualization, now in the separate
ITKVtkGlue repository). The level-set code itself is unchanged ITK 6 API.

Once they no longer render, nothing ties them to VTK, so they move from
Bridge/VtkGlue to Segmentation/LevelSetsv4, matching the ITK module.

What each example now writes

The two dense examples used to differ only in how they rendered, so each now
writes what its name describes:

example	output
VisualizeStaticDense2DLevelSetAsElevationMap	dense level-set values
VisualizeStaticDense2DLevelSetZeroSet	zero set, via ZeroCrossingImageFilter
VisualizeStatic{Whitaker,Shi,Malcolm}2DLevelSetLayers	sparse layers, sampled on the image grid

All five also fix a latent bug: the Otsu mask pipeline was never updated before
the level set was initialized, so the level set started from an empty mask and
every pixel landed outside it.

The rendered screenshot (levelsets.png) is removed from each page, since it
showed a renderer that no longer exists.

Verification

Built against ITK main: 0 build failures, and all 5 new tests pass. Each
output was inspected, not just checked for existence:

example	output
Dense ElevationMap	values −10.2 to 42.3, 2147 pixels inside
Dense ZeroSet	1914-pixel contour
Whitaker	layers −3, −2, −1, 0, 1, 2, 3
Shi	layers −3, −1, 1, 3
Malcolm	layers −1, 0, 1

Commit 1 is a pure move (25 renames at 100% similarity), so history follows the
files. pre-commit run --all-files passes.

The tests check that each example runs; they do not compare against a baseline
image, which would first need new test data published.

[dzenanz]

Looks good on a glance.

[dzenanz]

Some of Kitware's services are currently under DDoS attack. cmake.org timeout is real, but temporary.