feat(annotation): per-label + merged tiling previews, end-to-end

[clemsgrs]

Render tiling previews for annotation-aware sampling, mirroring the coordinate artifacts 1:1.

What changed

• PER_ANNOTATION: one tiling preview per active label, each drawn over that label's full-color mask backdrop with the selected tile grid, written under the per-annotation preview subdir (preview/tiling/{label}/{sample_id}.jpg).
• SINGLE_OUTPUT: one merged tiling preview over the multi-label mask backdrop with the union tile grid, at the flat preview location (preview/tiling/{sample_id}.jpg).
• Grid is the existing fixed black line (draw_grid default); no new grid-color config. Backdrop is drawn via write_coordinate_preview's mask-overlay path; the palette is built from the sampling pixel/color mapping.
• Preview subdir selection routes through the shared is_flattened_annotation helper (Prefactor: shared annotation→path flattening helper for artifact and preview paths #142), so preview paths match coordinate artifact paths exactly (a label literally named tissue, and the merged/None case → flat root).
• Rendered through the existing preview thread-pool executor (preview_executor / _finalize_pending_tiling_preview), one task per artifact, so renders never serialize on the compute hot path.
• Zero-tile labels skip their preview but keep a manifest row with num_tiles=0, empty tiling_preview_path, and a populated mask_preview_path.
• Identical under INDEPENDENT and JOINT selection; governed by preview.save_tiling_preview. The Annotation previews: multi-label mask preview, end-to-end #146 mask-preview behavior (repeated mask_preview_path) is unchanged.

Tests

• New structural tests at the tile_slides() seam in tests/test_annotation_coverage.py: per-label preview counts/paths (both selection strategies), merged single-output preview at flat root, per-row tiling_preview_path, zero-tile skip (row kept with num_tiles=0 / empty preview / populated mask preview), tissue/None flattening, and the no-preview path.
• Visual fixture in tests/test_overlay_semantics.py pinning the rendering: label mask backdrop under the fixed black grid line.

Acceptance criteria

• PER_ANNOTATION: one tiling preview per active label, full-color mask backdrop + tile grid; path matches the coordinate artifact path via the shared helper.
• SINGLE_OUTPUT: a single merged tiling preview over the multi-label mask backdrop with the union tile grid, at the flat preview location.
• Tile grid is the fixed black line; no new grid-color config introduced.
• Zero-tile labels skip their tiling preview but keep a manifest row with num_tiles=0, empty tiling_preview_path, populated mask_preview_path.
• Tiling previews are generated via the existing preview executor, not serialized on the compute hot path.
• Each row's tiling_preview_path is populated for non-empty results in both output modes.
• Behavior is identical under INDEPENDENT and JOINT selection.
• Structural tests at the tile_slides() seam cover both output modes; a visual fixture pins the rendering.

Closes #144

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