DC-PUMP-05

Cooperative keep-alive liveness under downstream backpressure. No stall of the downstream consumer -- of ANY duration or cause (block application, epoch-boundary fold, recovery-checkpoint capture) -- may prevent the wire pump from sending MsgKeepAlive on the DC-PUMP-03 cadence AND consuming the peer's echoed MsgResponseKeepAlive. The wait for downstream capacity is itself a select! (emit_cooperative: events_out.reserve() vs the cadence timer vs inbound), so the cadence is REACHABLE while parked. Servicing only the timer is INSUFFICIENT: the BLUE agency guard suppresses every later tick once the lane is ClientAwaiting, giving ~117s of tolerance against an unbounded stall. MUST NOT: await a bounded events_out.send outside the cooperative wait; make the frame handlers async (they could park on such a send); queue the cadence frame on outbox_payloads (drained only by the outer loop, so it would never reach the wire during the very stall it exists to survive).