Kafka gateway: Design Consumer offset persistence strategy for Kafka gateway Phase 2 #3540

Part of #3560.

Summary

Decide and document how the Kafka gateway stores consumer group offsets for Phase 2.

Motivation

Open question from #3253 (comment):

Phase 2 is blocked until this decision is recorded.

Depends on

• Kafka gateway: Kafka gateway Phase 1 — docker-compose, E2E tests, and quick-start docs #3539 — Phase 1 complete (or near-complete)

Deliverables

• ADR or design comment on feature - Order of Implementation discussion of kafka to iggy bridge #3253 with chosen option
• Data model: key = (group, topic, partition) → committed offset
• Durability, backup, multi-bridge-instance behavior documented
• Partition assignment strategy decision: range / round-robin / sticky vs Iggy balanced

Acceptance criteria

• Maintainers sign off on Option A/B/C
• Phase 2 implementation issues unblocked