test: qualify the migrated inference path - #39

Tracks #36 and is the third PR in the copick-easymode OME-Zarr 0.5 / Zarr v3 migration stack.

This completes the source-side validation path:

• locked Linux Python 3.11-3.13 and Windows Python 3.13 jobs;
• exact easymode source-commit private/public API validation;
• opt-in backend-capable real-model evidence tooling;
• commit-safe evidence records without absolute local paths or manual redaction;
• mandatory nonzero foreground before and after the saved segmentation round trip; and
• runtime validation of saved OME-Zarr 0.5 / Zarr v3 uint8 output.

The prior local CPU record was removed because its probability map produced an all-background segmentation and its committed schema had been edited by hand. Replacement evidence should be committed only after the corrected command succeeds unchanged on a real input that produces foreground.

Validation: all 40 source tests pass across the configured matrix, and all existing CI checks are green.

Remote S3/SSH/ML Croissant fault-injection evidence remains environment-owned and is documented rather than claimed as automated. No PyPI setup, release PR, or publication is included.