Kafka gateway: Kafka gateway IggyBridge core — SDK client and stream/topic mapping #3533

Part of #3560.

Summary

Implement the IggyBridge integration layer in gateways/kafka/ that connects decoded Kafka protocol handlers to the Iggy Rust SDK.

Motivation

Phase 1 of the Kafka → Iggy bridge rollout requires mapping Kafka topics/partitions to Iggy streams/topics and exposing produce/fetch/watermark operations. This module is shared by all Phase 1 API handlers.

Depends on

• Kafka gateway: Build Gateway to Iggy for existing Kafka users - Create a TCP Listener for Kafka users #3421 — TCP listener, request decode, ApiVersions firewall

Scope

• bridge/iggy_bridge.rs (or equivalent) with async Iggy TCP client lifecycle
• ensure_stream_and_topic(kafka_topic, partition_count) → Iggy stream + topic create-if-missing
• Topic name → Iggy stream/topic mapping config (TOML/env)
• Connection config: Iggy address, credentials (SecretString pattern)
• high_watermark(stream, topic, partition) for ListOffsets
• Error mapping: Iggy errors → Kafka error codes where applicable
• Unit tests with mock or harness; no panics on Iggy unreachable at handler boundary

Out of scope

• Produce/Fetch handler wiring (separate issues)
• Consumer group offset storage

Acceptance criteria

• Bridge module compiles and is invoked from integration test stub
• ensure_stream_and_topic idempotent on repeated calls
• README section documents mapping rules and config

References

• Rollout plan: feature - Order of Implementation discussion of kafka to iggy bridge #3253 (comment)
• Parent discussion: feature - Order of Implementation discussion of kafka to iggy bridge #3253
• Related: Feature comparison - Kafka vs Iggy for Enterprise consumption #3081
• Prior issue: Kafka gateway: Build Gateway to Iggy for existing Kafka users - Create a TCP Listener for Kafka users #3421