feat(mqtt): dispatch embedded inbound publishes from the session loop (design 054 §4.7)

[lxsaah]

Stage 9 of the 054 implementation plan. The embedded MQTT session delivers each inbound publish straight into its records from drain_packets, while it still holds the decoded packet. The event channel, MqttSource and the pump_source task are gone. Outbound still goes through the action channel and pump_sink until stage 10.

Change

• embedded/mod.rs:
  • build_plain and build_tls build InboundDispatch::new(db, "mqtt", &MqttGrammar) instead of inbound_router, and subscribe its subscriptions().
  • The dispatcher moves into the session task through setup_manager/setup_tls_manager and run_sessions/run_tls.
  • Removed: AimdbMqttEvent with its conversion, EventChannel, MqttSource, and the pump_source call.
  • Breaking: aimdb_mqtt_connector::embedded::AimdbMqttEvent was public.
• manager.rs: the EventChannel alias and the crate-private FromApplicationMessage trait are removed.
• session_loop.rs:
  • run_session, client_loop and drain_packets take &InboundDispatch, named dispatch because the raw-chunk channel is already called inbound.
  • A new deliver calls dispatch.dispatch(publish.topic_name(), publish.payload()) for Publish and PublishAndPuback. It keeps the empty-topic check, the liveness-only events and the DISCONNECT error. Received is gone.
  • feat(mqtt): encode embedded packets into one bbqueue write ring (design 054 §4.7) #283 review nit: PUBACK room is now waited for only when the head packet is a PUBLISH above QoS 0, read from its first byte (PacketReader::head_needs_ack), not before every packet.
• connector.rs: the Embedded rustdoc says what QoS 1 inbound means now: the PUBACK goes out before delivery, so it means "reached AimDB". A record whose buffer is full drops the message, and the broker does not resend.
• setup_tls_manager takes 8 arguments now and gets #[allow(clippy::too_many_arguments)], as the session loop's long signatures already do. Stage 10 replaces the action channel there.

Net: +170 / −171 in aimdb-mqtt-connector, tests included.

Tests

• New the_embedded_backend_dispatches_inbound_on_its_session_task (tokio_broker): with an inbound link and no outbound ones, build() returns one future. Against the previous code the same test sees 2, which I checked by running it on the old src/.
• New only_a_publish_above_qos_0_needs_an_ack (packet_reader).
• The session-size test builds an InboundDispatch over an empty database (NoopRuntimeOps) and still passes the ≤ 8,192-byte bound.

Existing suites, unchanged and passing:

Suite	Passed
session_loop (QoS 1 delivery, inbound flood, ping cadence)	6
tokio_broker (reconnect/resubscribe, round trip, Maximum Packet Size, new count test)	5
embassy_broker	1
tls_session	3
tls_broker	3
backend_parity	8
write_ring_proofs	1
--features std	40
embedded-tls lib	43

Verification

• Every aimdb-mqtt-connector clippy leg (host, test targets, thumbv7em including embassy-tls + defmt) and every doc leg (-D warnings) from the Makefile: clean. cargo fmt --all --check: clean.
• embassy-mqtt-connector-demo and weather-station-gamma build for thumbv8m.main-none-eabihf.
• CI does not run on PRs into feat/054-connector-boundary.

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