Fixes a bug in MultiTaskSegmentor’s WSI probability-map vertical merging when spilling intermediate probability rows to Zarr. Previously, once a head spilled to Zarr, subsequent chunks for that same head could continue accumulating in memory instead of being appended to the Zarr. At finalization, the code preferred the Zarr array and discarded the in-memory chunks, producing truncated probability maps for only the heads that spilled resulting in the shape mismatch that was being seen by me and Jiaqi.

The fix keeps hold of the opened Zarr group after the first spill and routes later chunks for that head into the same Zarr dataset. This preserves the full merged probability-map height across all heads.

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Co-authored-by: Shan E Ahmed Raza <13048456+shaneahmed@users.noreply.github.com>