BUG: Fix LabelMap and label-statistics correctness bugs from issue 6575

[hjmjohnson]

Fixes six LabelMap / label-statistics correctness bugs catalogued in #6575 (items B40–B45): one BUG: commit per item. All in-memory logic, no fixtures; three items ship dedicated fail-before/pass-after GoogleTests and the rest are covered by the module regression (136/136 local).

Per-item summary

Item	Site	Fix
B40	MergeLabelMapFilter (KEEP)	The collision-deferred deque was declared outside the per-input loop and never cleared, so with ≥3 inputs an object deferred on one input was pushed again on the next — aliasing one object under two label keys. Declare the deque inside the loop.
B41	LabelMapContourOverlayImageFilter (SLICE_CONTOUR)	The slice-radius guard tested the write cursor j against SliceDimension where it meant the read index i; SliceDimension == 0 left the radius all-zero and drew no contour. Test the read index.
B42	LabelMapMaskImageFilter (Crop)	An empty/zero-line object set left the bounding-box min/max sentinels untouched, so maxs - mins + 1 overflowed (UB) and the filter allocated a 2×2 image at index 2⁶³−1. Use an empty region when no lines are found (both crop sites).
B43	AttributeUniqueLabelMapFilter	The "remove objects without lines" pass used an inverted condition (while (it.IsAtEnd())) and never ran, so an object whose pixels were all claimed by a higher-priority object survived with zero lines (and fed the B42 crop path).
B44	LabelMap::PushLabelObject	On a completely full label range the gap scan found no free label but the "map full" test (label == lastLabel) never held, so AddLabelObject silently overwrote the object's own entry. Track whether a free label was found and throw otherwise.
B45	StatisticsLabelMapFilter	Weighted elongation/flatness used unclamped principal moments and one shared guard, so an indefinite weighted covariance from a signed feature image produced NaN where ShapeLabelMapFilter returns 0. Clamp the moments and guard each ratio, matching ShapeLabelMapFilter.

Testing

New itkLabelMapBugfixGTest.cxx adds three tests, each verified to fail on the unfixed tree and pass after:

• B44 — a full 8-bit label range makes PushLabelObject throw (silently overwrote before).
• B45 — a cross-shaped object with positive x-arm / negative y-arm feature weights gives an indefinite weighted covariance; weighted elongation/flatness stay finite (NaN before).
• B43 — two fully-overlapping objects: uniqueness empties one, which is then removed (count 2 → 1; stayed 2 before).

B40/B41/B42 are covered by the existing ITKLabelMap + ITKImageFusion regression (136/136 pass locally); dedicated fail-before tests for those can be added on request. pre-commit run --all-files is clean.

Findings and analysis by the #6575 reporter; see that ledger for the full derivations.

[hjmjohnson]

@greptile review

[greptile-apps]

Greptile Summary

This PR fixes several LabelMap and label-statistics correctness bugs. The main changes are:

• Correct slice contour radius mapping for the skipped slice dimension.
• Remove emptied label objects after uniqueness filtering.
• Throw when PushLabelObject cannot find a free label.
• Use an empty crop region when matching objects have no lines.
• Clear deferred merge objects per input.
• Clamp weighted principal moments and guard weighted shape ratios.
• Add LabelMap regression tests for the covered cases.

Confidence Score: 5/5

This looks safe to merge.

No blocking issues found in the changed code.

No files need attention.

T-Rex Logs

What T-Rex did

• The build-environment evidence shows cmake, ctest, ninja, and pixi are unavailable even though /usr/bin/c++ exists.
• An attempted CMake configure with the requested narrow flags ran and failed with 'cmake: command not found' and EXIT_CODE: 127.
• The wiring-evidence confirms the new GTest file exists, contains B43/B44/B45 test declarations, and is wired into ITKLabelMapGTests.

_{Ran code and verified through T-Rex}

Important Files Changed

Filename	Overview
Modules/Filtering/ImageFusion/include/itkLabelMapContourOverlayImageFilter.hxx	Corrects the contour thickness remapping to skip the selected slice dimension.
Modules/Filtering/LabelMap/include/itkAttributeUniqueLabelMapFilter.hxx	Runs the removal pass for label objects that become empty during uniqueness filtering.
Modules/Filtering/LabelMap/include/itkLabelMap.hxx	Adds explicit free-label tracking before adding a pushed label object.
Modules/Filtering/LabelMap/include/itkLabelMapMaskImageFilter.hxx	Builds a zero-size crop region when no object lines contribute to the crop box.
Modules/Filtering/LabelMap/include/itkMergeLabelMapFilter.hxx	Scopes deferred collision objects to each input during keep-mode merging.
Modules/Filtering/LabelMap/include/itkStatisticsLabelMapFilter.hxx	Matches weighted shape ratio handling to the clamped principal moment behavior.
Modules/Filtering/LabelMap/test/CMakeLists.txt	Registers the new LabelMap bugfix test source.
Modules/Filtering/LabelMap/test/itkLabelMapBugfixGTest.cxx	Adds regression tests for full label ranges, signed weighted shape metrics, and emptied unique-label objects.

_{Reviews (3): Last reviewed commit: "BUG: Clamp weighted moments in Statistic..." | Re-trigger Greptile}

[hjmjohnson]

@greptileai review this draft before I make it official