feat: add PushConsumer delivery loop and RedeliveryTracker (C3+C4)
C3 – PushConsumerEngine delivery dispatch:
- Add DeliverSubject property (mirrors consumer.go:1131 dsubj field)
- Add StartDeliveryLoop / StopDeliveryLoop: background Task that drains
ConsumerHandle.PushFrames and calls a sendMessage delegate per frame
- Delivery loop honours AvailableAtUtc for rate-limiting (consumer.go:5120)
- Data frames: HMSG headers Nats-Sequence, Nats-Time-Stamp, Nats-Subject
(stream.go:586 JSSequence / JSTimeStamp / JSSubject constants)
- Flow-control frames: "NATS/1.0 100 FlowControl Request" (consumer.go:5501)
- Heartbeat frames: "NATS/1.0 100 Idle Heartbeat" (consumer.go:5222)
- Add DeliverSubject field to ConsumerConfig (consumer.go:115)
C4 – RedeliveryTracker with backoff schedules:
- Schedule(seq, deliveryCount, ackWaitMs): computes deadline using backoff
array indexed by (deliveryCount-1), clamped at last entry (consumer.go:5540)
- GetDue(): returns sequences whose deadline has passed
- Acknowledge(seq): removes sequence from tracking
- IsMaxDeliveries(seq, maxDeliver): checks threshold for drop decision
- Empty backoff array falls back to ackWaitMs
Tests: 7 PushConsumerDelivery tests + 10 RedeliveryTracker tests (17 total)
612b15c781