- `detections_from_xml_obj` now builds `np.empty((0, H, W))` for a background image under `force_masks=True` instead of letting `np.array([])` collapse to shape `(0,)`, which failed `Detections` mask validation
- document the forced `class_id` `dtype=int` with an inline comment and state the integer-dtype guarantee in the `detections_from_xml_obj` docstring Returns section
- add background-image coverage: force_masks empty 3D mask, all-background dataset, background-first ordering, and save-then-load round-trip
- add changelog entry for the `force_masks=True` background-image mask fix
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Co-authored-by: claude[bot] <209825114+claude[bot]@users.noreply.github.com>
Co-authored-by: OpenAI Codex <codex@openai.com>
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