BUG: Propagate rank to the last axis in FastApproximateRank

[physwkim]

Addresses item B18 of #6575.

FastApproximateRankImageFilter is a separable mini-pipeline of one per-axis RankImageFilter per image dimension. Its SetRank() forwards the caller's rank to only the first ImageDimension - 1 sub-filters:

for (unsigned int i = 0; i < TInputImage::ImageDimension - 1; ++i)
{
  this->m_Filters[i]->SetRank(m_Rank);
}

so the last axis always runs at the sub-filter default of 0.5 — a median — regardless of the requested rank, and for 1-D images the loop body never runs at all. GetRank() still reports the requested value, so the mismatch is silent. The fix is the loop bound: ImageDimension - 1 → ImageDimension (MiniPipelineSeparableImageFilter holds exactly ImageDimension sub-filters, one per axis).

The new regression test isolates the last axis: a 7×7 image constant along axis 0 and strictly increasing along axis 1, radius (1, 1), rank 0.02. The axis-0 pass is an identity on constant data, so the axis-1 (last-axis) pass is the discriminator: per RankHistogram's selection rule (target = rank * (entries - 1) + 1), rank 0.02 over a 3-sample window selects the minimum while the default 0.5 selects the median. Interior pixels must equal the window minimum 100·y; unfixed, they come out as the median 100·(y+1).

Verified locally (Linux, GCC 13, Release): the new test fails on unmodified main (interior pixels 200, expected 100 — the last axis still ran at rank 0.5) and passes with the fix; the full ITKReview suite plus the ITKImageStatistics, ITKLabelVoting, ITKDisplacementField, and ITKRegionGrowing suites pass with the fix applied.

PR Checklist

• No API changes were made (or the changes have been approved)
• No major design changes were made (or the changes have been approved)
• Added test (or behavior not changed)
• Updated API documentation (or API not changed)
• Added license to new files (if any)
• Added Python wrapping to new files (if any) as described in ITK Software Guide Section 9.5
• Added ITK examples for all new major features (if any)

AI assistance

• Tool: Claude Code (claude-fable-5, with claude-opus-4-8 subagents)
• Role: implemented the fix and regression test from the analysis previously filed as item B18 of BUG: Findings collected while porting ~60 filters — reported together in one issue #6575; derived the expected test values from RankHistogram's selection rule before implementation
• Testing: built locally and verified the regression test fails before / passes after the fix; five module test suites pass with the fix
• Note: clang-format 19.1.7 was not available locally; formatting was hand-matched to ITK style and may need the CI formatter's touch-up