Function set_edge_group_policy 

pub async fn set_edge_group_policy(
    pool: &PgPool,
    partition_config: &PartitionConfig,
    flow_id: &FlowId,
    downstream_execution_id: &ExecutionId,
    policy: EdgeDependencyPolicy,
) -> Result<SetEdgeGroupPolicyResult, EngineError>

set_edge_group_policy — RFC-016 Stage A.

Reject when at least one edge has already been staged for downstream_execution_id (the ordering rule: policy must be set BEFORE the first add_dependency). Parity with the Valkey Lua path, which returns invalid_input → EngineError::Validation in the same situation (not Conflict, per in-tree behavior).

Stage A supports AllOf fully; AnyOf / Quorum flow through the same write path (Stage B’s resolver is backend-specific — Valkey’s Lua honours them, Postgres Wave 4c stores them and Wave 5 wires the resolver). Callers that need the Stage-A ordering check can rely on this method regardless of the variant.