nx_mqtt_sink_for_telemetry.nx

nx_mqtt_sink_for_telemetry.nx -- MQTT sink for telemetry events. Converts an NxTelemetryEvent into an MQTT PUBLISH frame, so the substrate's anomaly verdicts (pred-maint commit d3037668 + telemetry bus 469f822f) become operator-visible on whatever MQTT broker the operator points to. Architectural note (V1 vs V2): V1 ships a STANDALONE sink module. The operator calls nx_mqtt_sink_emit alongside the bus's stdout-JSON sink in a fanout pattern (manual wiring): nx_tm_emit(bus, event) -- writes stdout NDJSON nx_mqtt_sink_emit(cfg, event, ...) -- builds PUBLISH frame that operator transmits over socket V2 will extend NxTelemetrySink with a sealed sink-kind enum (STDOUT_JSON / MQTT / SYSLOG / FILE / FANOUT) so the bus dispatches automatically; this V1 module survives as the MQTT-specific frame builder either way. Status: SEED v0.1.0. 2026-05-27. WINNER-TIER: WINNER-A CANDIDATE (preserved from telemetry bus; this commit adds the MQTT-output capability without changing the underlying value-add claim) INCUMBENTS: Eclipse Paho's MQTTClient_publishMessage, mqtt.js client.publish(), AWS IoT SDK publish(), Mosquitto's mosquitto_pub CLI NUMBERS: V1 ships the frame-builder; latency vs Paho publish() pending paired bench on real broker GAP: Paho buffers + retries + auto-reconnects internally; V1 is purely the wire-format builder. Operator owns socket lifecycle for V1; V2 adds nx_tcp_shim integration when that ships. PLAN: M-next: paired publish-latency bench vs Paho; add socket-write integration when nx_tcp_shim ships EXEMPTION REASON: n/a; provisional pending measurement Topic shape (operator-customizable via topic_prefix): <prefix>/<source> where <source> is the NxTelemetryEvent.source_str (e.g.,