Respect QuPath ROI Z/T planes when exporting selected areas and RegionOf masks #21

Confirmed contract: QuPath 0.7.0 preserves ROI image planes

QuPath ROIs carry their own Z and T coordinates. QuPath's RegionRequest.createInstance(path, downsample, Collection<ROI>) throws IllegalArgumentException for ROIs on different ImagePlanes:

• Official QuPath Javadoc (currently published as 0.6.0; verified separately against 0.7.0 source): ROI.getImagePlane(), getZ(), getT()
• Official QuPath Javadoc (currently published as 0.6.0; verified separately against 0.7.0 source): RegionRequest collection overload
• QuPath v0.7.0 source, RegionRequest.java lines 116–138

LabConstrictor deviation

The extension's selectionArea() combines all selected annotation bounds without checking ROI.imagePlane, then assigns the viewer's Z/T to the exported rectangle. selectionMask() similarly paints all selected 2D ROI shapes into the same mask. The exported image, however, is requested from one Z/T plane via LcArea.request().

A selected annotation on Z=1 can therefore define the crop or supply a label in an image exported at viewer Z=0. Multiple selected annotations from different time points are likewise flattened into one exported mask without indicating that they refer to different planes. This is a scientific-data mismatch, not merely a display issue.

Proposed fix

Apply an explicit image-plane policy consistently to area selection and RegionOf masks. Prefer to reject mixed Z/T selections with an actionable message, or filter to the current viewer plane with clear UI indication. If single-plane selection does not match the viewer, either switch to the annotation's plane deliberately or refuse it. Do not silently mix planes.

Acceptance criteria

• Single annotation at viewer Z/T is exported correctly.
• Annotation at a different Z or T cannot silently define a crop/mask for the current plane.
• Two annotations with different Z or T are rejected or explicitly filtered according to a documented policy.
• The exported TIFF's plane, generated region mask and restored result annotations all use the same image plane.
• Add QuPath 0.7 GUI fixture with two planes and scripted selection across planes; check exported pixel values and annotations.

Priority: P1 — possible incorrect scientific image/region pairing. Verified against the actual extension and QuPath v0.7.0 Java source. A running QuPath regression test is still needed.