Add QoS2 lifecycle hooks to expose full message delivery states

[manuio]

Background

mproxy's Handler interface exposes hooks for CONNECT, PUBLISH, SUBSCRIBE, UNSUBSCRIBE, and DISCONNECT. For QoS2 flows, the full 4-step handshake between broker and client is invisible to the handler — packets either fall through notify's default case (upstream) or notify is never called at all (downstream).

This makes it impossible to track per-device, per-message delivery state at the application level.

QoS2 flow (broker → device):

broker → device : PUBLISH    (downstream — no hook)
device → broker : PUBREC     (upstream   — falls through notify)
broker → device : PUBREL     (downstream — no hook)
device → broker : PUBCOMP    (upstream   — falls through notify)

Proposal

Extend the Handler interface with four new hooks:

// PubSent is called when broker dispatches a QoS2 PUBLISH to a client.
PubSent(client *Client, packetID uint16, topic string)

// PubReceived is called when a client sends PUBREC.
PubReceived(client *Client, packetID uint16)

// PubRelease is called when broker sends PUBREL to a client.
PubRelease(client *Client, packetID uint16)

// PubComplete is called when a client sends PUBCOMP.
PubComplete(client *Client, packetID uint16)

Wire in session.go:

// upstream (notify)
case *packets.PubrecPacket:
    s.handler.PubReceived(&s.Client, p.MessageID)
case *packets.PubcompPacket:
    s.handler.PubComplete(&s.Client, p.MessageID)

// downstream (new notifyDown)
case *packets.PublishPacket:
    s.handler.PubSent(&s.Client, p.MessageID, p.TopicName)
case *packets.PubrelPacket:
    s.handler.PubRelease(&s.Client, p.MessageID)

Use Case

Full per-device, per-message delivery lifecycle:

Step	Direction	Hook	Meaning
1	broker → device	PubSent	Message dispatched
2	device → broker	PubReceived	Device acknowledged receipt
3	broker → device	PubRelease	Broker releasing
4	device → broker	PubComplete	Device confirmed delivery ✓

State stored keyed by clientID:packetID, exposable via API or event stream — no application-level ACK topic needed.

Breaking Change

Breaking change to the Handler interface. All existing implementations must add the four new methods.