A malformed OME-Zarr labels group is dropped with an info log #38

Data-flow audit 2026-10-29 (H1), MAJOR. At v0.3.1.

What happens: masks.py:319-323 catches the error from reading an OME-Zarr labels group, logs it at info level and continues.

Failure: if every image of a mask type is affected, the type disappears from the inventory and the mask_missing warning never fires, so those objects are silently not measured.

Fix: report it as an issue (for example unreadable_mask, an error), never only a log line.