BUG: Backport Voronoi infinite loop fix to release-5.4

[hjmjohnson]

Backport 3b3f3c24d4 from main to release-5.4. Fixes an infinite loop in VoronoiDiagram2DGenerator::ConstructDiagram when Fortune's algorithm produces degenerate near-zero-length edges.

Part of #6051 (backport candidates for 5.4.6).

Cherry-pick details

Cherry-picked from commit 3b3f3c24d4 (merged to main via PR #6017). Required manual conflict resolution:

• itkVoronoiDiagram2DGenerator.hxx: moved frontbnd/backbnd declarations from function scope into block scope (release-5.4 had them at outer scope; main moved them to inner loop + second usage block)
• CMakeLists.txt: added new test file to the test list while preserving release-5.4 formatting style

Local build and test verified: 8/8 Voronoi tests pass including the new itkVoronoiDiagram2DInfiniteLoopTest.

[greptile-apps]

Greptile Summary

This PR backports commit 3b3f3c24d4 from main to release-5.4, fixing an infinite loop in VoronoiDiagram2DGenerator::ConstructDiagram triggered by near-collinear seeds that cause Fortune's algorithm to emit near-zero-length edges with distinct vertex IDs but geometrically identical endpoints. The fix introduces a stall counter to detect when chain assembly makes no forward progress and adds explicit degenerate-edge detection using the existing differentPoint() / DIFF_TOLERENCE check, accompanied by a new regression test.

Confidence Score: 5/5

Safe to merge — the fix is logically sound, preserves all existing retry semantics, and converts a guaranteed hang into a clear exception for truly stuck configurations.

All remaining findings are P2 style suggestions. The stall counter correctly terminates after one complete no-progress pass (matching the original infinite-loop condition), the degenerate-edge discard reuses the pre-existing DIFF_TOLERENCE check, and the exception guard is strictly better than the previous silent hang. The conflict resolution (variable scope) is correct, and the regression test directly reproduces the reported bug.

No files require special attention.

Important Files Changed

Filename	Overview
Modules/Segmentation/Voronoi/include/itkVoronoiDiagram2DGenerator.hxx	Core fix: stall counter + degenerate-edge discard replaces the previously unbounded retry loop; variable scoping conflict with release-5.4 resolved correctly via block-scope redeclarations of frontbnd/backbnd.
Modules/Segmentation/Voronoi/test/itkVoronoiDiagram2DInfiniteLoopTest.cxx	New regression test with hardcoded near-collinear seeds that previously caused an infinite loop; uses ITK_TRY_EXPECT_NO_EXCEPTION and verifies all 6 cell boundaries are non-empty.
Modules/Segmentation/Voronoi/test/CMakeLists.txt	Adds new test source and itk_add_test() registration; minor formatting inconsistency in the NAME keyword layout compared to the rest of the file.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[ConstructDiagram: pop first edge → seed chain] --> B[remainingBeforeStall = deque.size]
    B --> C{deque empty\nor stall=0?}
    C -- No --> D[pop edge from deque\ndecrement stall counter]
    D --> E{edgeStart ≈ edgeEnd\ndifferentPoint = false?}
    E -- Yes: degenerate --> F[discard edge\nreset stall counter]
    F --> C
    E -- No: valid edge --> G{Can attach to\nchain front/back?}
    G -- Yes --> H[attach edge\nupdate front/back\nreset stall counter]
    H --> C
    G -- No: boundary bridge? --> I{Boundary ID\nmatch?}
    I -- Yes --> J[insert bridge + edge\nreset stall counter]
    J --> C
    I -- No --> K[push edge back\nno stall reset]
    K --> C
    C -- Yes: done --> L{deque empty?}
    L -- Yes --> M[close chain if needed\nbuild cell]
    L -- No: stuck edges remain --> N[itkExceptionMacro\nunexpected configuration]

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_{Reviews (1): Last reviewed commit: "BUG: Fix infinite loop in VoronoiDiagram..." | Re-trigger Greptile}

[hjmjohnson]

CI failures are known flakes — not related to this PR's code changes.

• ITK.macOS.Python: exit code 255 from ci_completed_successfully in the dashboard script (treats warnings as fatal). CDash shows 0 compile errors and 0 test failures.
• CDash: status check is stale/pending — actual CDash results are clean.

All 8 Voronoi tests pass locally (verified on macOS ARM64). The spell check that previously failed (unattachable not in dictionary) was fixed in the current HEAD.

[hjmjohnson]

/azp run ITK.macOS.Python