Fix black previews of off-center masks and >4 GiB .nii.gz expansion

[marcoduering]

Summary

1. Masks whose foreground misses the center slices rendered black everywhere

The segmentation LUT and the intensity window were derived only from the three center slices. For a lesion or single-ROI mask that misses all three, the labels were empty (no LUT) and the window was Bounds(0,0), so every slice rendered black, and window/level drag was inert (_winRefSpan == 0).

• MiqVolume.DetectSegmentationLut (shared by the initial load and post-expansion): when the center sample is all background, collect labels from all of volume 0. A single label gives the white mask LUT; several give a colour LUT.
• MiqVolume.WindowFromSlices: a null or degenerate center-slice window falls back to VolumeWindow, a window over the whole volume at that timepoint (two passes, at most 1M strided samples), widened to include the background value. Used by CenterInteractiveState and the per-volume SharedWindow.
• MiqTriPlanarControl: the W/L reference span falls back to a non-zero value, so right-drag works on a constant volume.

2. .nii.gz over 4 GiB uncompressed was offered an expansion that cannot succeed

ParsePartial decided ExpansionBlocked from the gzip ISIZE footer, which is the size mod 2^32. Example: a 91×109×91×1200 float32 series (4,332,619,552 B) reports ISIZE 37,652,256, so it was not blocked. The first scrub then decompressed toward 2 GB, threw, and the swallowed error left "loading…" on screen permanently.

• Blocking now uses the header-derived size (MiqParser.NiftiFileExtent: vox_offset + every dim × bytes per voxel). ISIZE is no longer used in ParsePartial.
• A stream that ends inside volume 0 goes to the full parse, which reports the truncation.

Behaviour changes

• A volume whose center slices are all NaN, but which has finite data elsewhere, now shows an image instead of the "No finite voxel values" message.
• A multi-volume .nii.gz that holds exactly one volume of data now loads as a partial preview instead of a full parse. Missing volumes render black, as they already did in the full parse.

Tests

• SegmentationDetectionTests: off-center binary mask (white LUT), off-center multi-label (colour LUT), window fallback with the Off setting (window [0,1], mask pixels visible), all-zero volume stays degenerate.
• GzipPartialBlockingTests (non-parallel collection, because it lowers MaxArrayBytes): wrapped ISIZE with too-large header is blocked, fitting header is expandable, few-volume too-large file stays blocked, truncated volume 0 throws, NiftiFileExtent.

Not yet built or run: the authoring environment had no .NET SDK (download blocked by network policy), and CI builds only the plugin, not MIQ.Tests. Please run dotnet build and dotnet test locally before merging.

🤖 Generated with Claude Code

https://claude.ai/code/session_01XqBwhnTRsqxk3SEvp9z4T9

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Generated by Claude Code