perf(mqtt): publish a write batch concurrently, serialised per topic; writes feed health (R06:M-8)

WriteAsync published one awaited QoS 1 round-trip per request, so 50 writes to a
stalled broker cost 50 publish deadlines. Different topics now publish concurrently
(at most 8 in flight); same-topic writes stay in request order. A caller cancel still
fills every slot. A successful publish advances LastSuccessfulRead. MiniBroker now
frames packets properly and records/PUBACKs PUBLISHes.

Refs #744
Claude-Session: https://claude.ai/code/session_01Ub9hGmYLQvn9XETzw7CMsT

docs/drivers/Mqtt.md

@@ -491,6 +491,11 @@ with their real type; a birth type the encoder cannot represent (any `*Array`, D
 matters and is deliberately not configurable: a retained command would be replayed by the broker to the device on *every* future
 subscribe — including the device's own reconnect — so one stale write would be re-applied forever.
 
+**A batch publishes concurrently, one topic at a time (R06:M-8).** Writes to *different* topics in one
+`WriteAsync` go out together, at most 8 in flight, so a stalled broker costs a batch about one publish deadline
+rather than one per request; writes to the *same* topic stay sequential in request order, so the last one wins.
+A successful publish (PUBACK, or a sent QoS 0 command) advances `LastSuccessfulRead` like an inbound message.
+
 **Plain QoS 1 is at-least-once.** Know the consequences before pointing a Plain `writeTopic` at a non-idempotent
 device topic: