Replace the R&D-activity split on every live item in item_ids with the same apportionments. Each changed item is flipped to R&D-eligible and gets manual_override=True.

Mirrors :func:bulk_update_items_by_ids for auth/scoping: the claim_id-scoped query is the authorization boundary (ids on another claim match nothing). Synthetic salary ids and real salary-category rows are skipped (employees are apportioned on the Salary tab), as are already-removed, linked refund rows (derived — they follow their original via the mirror), or unknown ids — so updated_count reflects only real changes. Items whose split already equals the requested one are left untouched (idempotent).