Kafka gateway: Kafka Produce API (key 0) — bridge to Iggy send_messages #3535

Part of #3560.

Summary

Wire Kafka Produce (API key 0) to IggyBridge::produce() — decode RecordBatch payloads and send to Iggy with balanced partitioning.

Motivation

Core write path for Kafka migration: existing producers must work without code changes.

Depends on

• Kafka gateway: Build Gateway to Iggy for existing Kafka users - Create a TCP Listener for Kafka users #3421
• Kafka gateway: Kafka gateway IggyBridge core — SDK client and stream/topic mapping #3533 — IggyBridge core

Scope

• Decode Produce v3–v9 (scoped range); preserve RecordBatch as opaque bytes where possible (hot path)
• Map Kafka topic + partition → Iggy stream/topic/partition
• Partitioning: balanced / keyed per Kafka record (document semantics)
• Produce response with correct base offset / error per partition
• KAFKA-18659: ApiVersions advertises Produce min=0 for librdkafka compatibility (encode layer)
• Integration test: produce via gateway → verify message in Iggy (SDK or CLI)

Acceptance criteria

• kafka-console-producer.sh or kcat -P can send to a topic through the gateway
• Messages visible in Iggy stream/topic
• At-least-once documented (Phase 1)

References

• Rollout plan: feature - Order of Implementation discussion of kafka to iggy bridge #3253 (comment)