/** * FieldRow — a single column row inside an EntityNode. * * Reference: `docs/plans/20260501-universal-wysiwyg-schema-ux-v1.00W.md` * §3.6 item 1 (column-level handles). * * Each row exposes FOUR React Flow Handles (left-target, left-source, * right-target, right-source) so the user can drag from any column to any * other column's handle. Handle IDs follow the canonical pattern * `${entityId}.${fieldId}.${side}-${kind}` so SchemaCanvas#onConnect can parse * them back into `source_field_id` / `target_field_id` on the relation row. * * Phase 2 edit affordance: * - A real `