DrylMultiAgentRun

Observable handle to a multi-agent flow (see CancellationToken) / CancellationToken)). The shared run surface aggregates the flow: State mirrors the active step, Text / TextStream carry the final agent's answer (sequential flows only), Usage sums the steps, and Error reports the first failing step (with its name as Source). Per-step detail lives on Steps — render with <c>DrylHandoffTrace</c>.