Updates mutable fields on existing v2 client record relations identified by (from, to, relationRole). Only direct, positions, and data may be modified; from, to, relationRole, createdAt, and updatedAt are immutable. The data field is shallow-merged with the stored value (provided keys overwrite, existing unspecified keys are preserved). Each item must include at least one of direct, positions, or data. All relations in the batch are verified to exist before any updates are applied; if any relation is missing, the entire request is rejected without persisting changes. Updates then run sequentially (best-effort after verification).