Kafka gateway: Kafka gateway Phase 1 — docker-compose, E2E tests, and quick-start docs #3539

Part of #3560.

Summary

Ship Phase 1 MVP: standalone gateway binary, Docker Compose (Iggy + bridge), E2E validation, and quick-start documentation.

Motivation

Phase 1 deliverables from #3253 (comment):

1. Standalone proxy binary (cargo build --release)
2. Docker Compose (Iggy server + Kafka bridge)
3. E2E with kafka-console-producer.sh / kafka-console-consumer.sh
4. README quick-start

Depends on

• Kafka gateway: Build Gateway to Iggy for existing Kafka users - Create a TCP Listener for Kafka users #3421
• Kafka gateway: Kafka Metadata API (key 3) — broker and topic discovery via Iggy #3534, Kafka gateway: Kafka Produce API (key 0) — bridge to Iggy send_messages #3535, Kafka gateway: Kafka Fetch API (key 1) — bridge to Iggy offset-based polling #3536, Kafka gateway: Kafka ListOffsets API (key 2) — earliest/latest via Iggy high watermark #3537, Kafka gateway: Kafka CreateTopics API (key 19) — Iggy stream/topic provisioning #3538

Scope

• docker-compose.yml under gateways/kafka/ (or examples/)
• CI job: build gateway + run scoped integration/E2E tests
• Manual test checklist aligned with gateways/kafka/docs/MANUAL_TESTING.md
• README: architecture diagram (sidecar), ports, config, limitations (no consumer groups in P1)
• Success metrics documented

Acceptance criteria

• New contributor can follow README and produce/consume in under 15 minutes
• All Phase 1 API keys pass automated tests

Tracking

• Discussion: feature - Order of Implementation discussion of kafka to iggy bridge #3253