Abstraction to call the pipeline.

One budget (FX-006): call/3 and call!/3 start the clock at the public entry — queueing behind a busy client and the pipeline's execution share one absolute deadline, and the remaining budget is handed to the engine rather than a fresh default. Expected request failures (a missed deadline, a dead pipeline) raise as FlowExtra.PipelineError at the caller boundary without killing the reusable client; unexpected implementation failures crash it normally. Admission answers are expected request failures too (FX-001 closure): call/3 raises FlowExtra.AdmissionError the same way, and cast/2 reports the pipeline's own answer — :ok from the client means the pipeline accepted and accounted the work, a definite refusal arrives as {:error, :overloaded | :unavailable}, a lost admission endpoint arrives as {:error, :noprocess} (communication with the admission owner failed; whether the submission executed or produced effects before that failure is not established), and an outcome learned only by its acknowledgment timing out arrives as {:error, {:unacknowledged, ref}} — uncertain, never spelled as a refusal. See FlowExtra.Admission.submit/3 for what each shape lets the caller infer.