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

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 https://github.com/apache/iggy/discussions/3253#discussioncomment-17181030:
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

- #3421
- #3534, #3535, #3536, #3537, #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: https://github.com/apache/iggy/discussions/3253