[marii-moe]

Split model claim into atomic facts

Summary

• Reviewers working in MODEL_LABEL_CORRECTION mode can now split a single model-generated claim into two or more atomic human claims directly from the review UI
• A new Split into atoms button appears inline with the existing review controls (Looks good, Extraction issue, Duplicate, Multiple Facts)
• Clicking it opens an inline editor inside the claim card — no modal, so the source response text stays visible and selectable alongside the editor
• Each atom gets its own text field, span selector, and importance grade
• Selecting spans via the existing staged-span mechanism auto-populates the atom text field; the reviewer can then edit it to refine wording
• Confirming the split automatically marks the original claim as Multiple Facts and adds the atoms as human claims tagged with split_from_position, preserving a traceable link back to the source claim
• The split_from_position field is validated server-side: it must reference a position that exists in the item's model claims

Schema change

HumanClaim gains an optional split_from_position: int | None field (defaults to null). Existing reviews are unaffected. The TypeScript client was regenerated.

[marii-moe]

Change was manually tested by splitting model generated atoms into more atoms. Also spot checked existing features to make sure they were not affected.