Create an edge

Creates a single typed edge from source to target. Edges are first-class — they carry their own id, properties, and audit trail; they are not embedded references on the item. Source and target must both exist in the caller’s tenant.

Edge writes are dual-gated: the caller needs both write permission on the source item’s type AND write permission on the edge type. Cardinalities and source/target type constraints are enforced at create time — violations return 400 edge_constraint_violation. Cycles on parent-of and supersedes are rejected with 400 edge_cycle. See Permissions — dual-gate and Edges.