Kafka gateway: Kafka Fetch API (key 1) — bridge to Iggy offset-based polling #3536

Part of #3560.

Summary

Wire Kafka Fetch (API key 1) to IggyBridge::fetch() — return messages from Iggy by partition offset without consumer groups (Phase 1 assign/manual offset).

Motivation

Enables simple consumers and kafka-console-consumer.sh without --group for Phase 1 MVP.

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
• Kafka gateway: Kafka Produce API (key 0) — bridge to Iggy send_messages #3535 — Produce path (for E2E data)

Scope

• Decode Fetch v4–v12 (scoped range)
• Map fetch offset + max bytes → Iggy poll/fetch by offset
• Encode RecordBatch response (opaque bytes from Iggy payload)
• High-watermark / log end offset in response
• Integration test: produce then fetch same partition/offset

Acceptance criteria

• kafka-console-consumer.sh --from-beginning --partition 0 works without consumer group (Phase 1)
• Offset progression matches Iggy partition offsets

References

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