fix(annotation): read tiling-preview mask backdrop via reader, not WSI

[clemsgrs]

Summary

Annotation-aware tiling previews crashed on every real-slide run with 'WSI' object has no attribute 'read_level', so no preview/tiling/ JPEG was ever produced (the mask preview rendered fine). write_coordinate_preview built the mask backdrop as a WSI and passed it into draw_grid_from_coordinates → read_aligned_mask, which calls mask_obj.read_level(...)/mask_obj.spacings — reader/backend methods that a WSI does not expose. Introduced by #147.

Fix

Pass the backend reader (WSI(...).reader) into the grid renderer's mask-overlay path, mirroring the mask-preview path (overlay_mask_on_slide already uses mask_obj=mask_object.reader). The reader exposes read_level/spacings/level_downsamples, satisfying read_aligned_mask's contract. One-line behavioral change in write_coordinate_preview.

Regression test

tests/test_tiling_preview_mask_overlay.py drives the real openslide-backed fixture slide (tests/fixtures/input/test-wsi.tif) with a derived multi-label pyramidal mask (tumor=1 / stroma=2, split from test-mask.tif) through tile_slides with save_tiling_preview enabled, in both PER_ANNOTATION and SINGLE_OUTPUT modes, asserting a preview/tiling/*.jpg is actually written and the mask preview still renders. It fails on main (reproducing the crash) and passes after the fix. Wired into the CI integration step (pr-test.yaml).

The pre-existing test_overlay_semantics.py fake was the exact "fake WSI that exposes read_level" pattern that hid this bug; its fake now exposes read_level/spacings only on .reader, so a reader-less fake can no longer mask the gap.

Acceptance criteria

• Annotation tiling previews render on a real (openslide-backed) slide in both PER_ANNOTATION and SINGLE_OUTPUT modes — a JPEG is written under preview/tiling/ mirroring each coordinate artifact path.
• No read_level AttributeError; the mask backdrop is read via a reader, not a WSI.
• Mask-preview behavior is unchanged (still rendered).
• A regression test exercises the tiling-preview mask-overlay branch against a real backend whose WSI does NOT expose read_level; it fails on current main and passes after the fix.

Test result

Full default fast suite: 289 passed, 3 skipped, 46 deselected. Integration set (incl. new test): 6 passed.

Closes #148

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