Buy a size split across both legs of an approved cross-venue market link

Executes the plan GET /orders/route-quote previews: creates a parent route row and submits one child order per leg. Children go through the same validation, admission and rate limiting as POST /orders. Partial success is a 200. status is always submitting; a leg that failed to submit has order_id: null and a populated legs[].error. Track the parent via GET /orders/routes and the children via GET /orders/{order_id}. Feature-gated. Disabled unless ROUTED_EXECUTION_ENABLED is set on the deployment; otherwise every call is a 403. Auth & scope. Requires the mutation gate (API-key triple satisfies it). This handler enforces no scope of its own — each child order carries the normal trade:execute and per-provider checks.