Zero-allocation connector boundary (design 054)

[lxsaah]

Design 054, the zero-allocation connector boundary (docs/design/054-zero-alloc-connector-boundary.md). It lands stages 3–16, each already reviewed as its own PR into feat/054-connector-boundary (#277–#295).

Breaking. Stage 17, the CHANGELOGs and user-facing docs, follows after the final review of this PR.

What changes

Connectors no longer get per-route pump tasks from core. Each connector builds two objects in build() and moves them into its own transport task:

• InboundDispatch (new(db, scheme, grammar)) — dispatch(topic, payload) deserializes and produces into every matching record. It is synchronous, never blocks, and does not allocate for exact, pattern or known-key topics.
• OutboundRoutes (new(db, scheme)) — the transport pulls with next(), or with poll_stage + take_staged inside a select, when it can send.
  • A lock-free ready set means only routes whose readers woke are polled, in round-robin order.
  • Payloads go into one scratch buffer allocated at build time (OutboundPayload::Borrowed); owned serializers fall back to Owned.
  • RouteStats counts sent, rejected, lagged, topic-overflow and serialize-failed per route.
• TopicWriter / with_topic_writer / with_topic_fn — write the topic into a bounded TopicBuf and replace TopicProvider.

Per crate

• aimdb-core:
  • New: InboundDispatch, OutboundRoutes (plus RouteInfo, OutboundMessage, OutboundPayload, RouteStats), TopicWriter/TopicBuf/TopicOverflow, Reader::poll_recv.
  • pump_client now pulls from OutboundRoutes in one task.
  • Removed: Source, pump_source, pump_sink, Connector, the Serialized* types, TopicProvider, with_topic_provider, collect_outbound_routes/OutboundRoute.
  • Now crate-private: Router and inbound_router.
• aimdb-mqtt-connector:
  • Embedded backend: encodes every packet into one bbqueue write ring, sized at build (with_write_buffer, default 4096). Inbound PUBLISHes are dispatched from the session loop and outbound values are pulled from OutboundRoutes.
    • CONNECT advertises Maximum Packet Size.
    • Size checks at build cover routes, CONNECT and SUBSCRIBE, and an invalid qos/retain fails the build.
  • Native backend: one event loop plus one publish loop. The broker password is masked in error lines.
• aimdb-websocket-connector: the server dispatches client writes and pulls broadcasts in one task.
• aimdb-knx-connector:
  • The connection task dispatches telegrams and pulls group writes itself, and only sends while the tunnel is connected.
  • Breaking: KnxConnector::new(binder, delay, gateway_url); Channels and the related types are gone.
  • critical-section-std-impl is a deprecated no-op.
• aimdb-embassy-adapter: the EmbassySink/EmbassySource bridges are removed; into_box_future and NetStack stay.
• Demos, aimdb-codegen templates and docs: updated to the new constructors.

Measured

b0_alloc_connector, gated by make bench-gate in CI:

Row	Before	After
inbound dispatch: exact, pattern, known key	0	0
inbound dispatch, new key	1	1
outbound, static topic	2	0
outbound, written topic (was TopicProvider)	3	0
outbound, owned serializer	3	1
outbound, 8 routes round-robin / parked pull	—	0

MQTT round trip (alloc_round_trip.rs): the embedded backend allocates 0 times per round trip; the native backend stays at or below 11 (rumqttc).

Testing

• CI: the full matrix passed on the stage 16 head (refactor(core)!: remove Source, pump_sink, Connector and TopicProvider (design 054) #295 while it targeted main).
• Live: the stage 12 runs against mosquitto passed, covering plain, password, native-tls and rustls, plus the weather mesh.
• Not yet run: the Embassy examples on hardware.

Before merging

• Stage 17: CHANGELOG entries for every affected crate, with the migration table, and the user-facing docs.

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