Kafka gateway: Kafka DeleteRecords — trim topic to offset via Iggy retention

Part of #3560.

## Summary

Implement **DeleteRecords (21)** — trim topic/log to a specific offset, mapped to Iggy retention or explicit delete where supported.

## Depends on

- #3546 — DeleteTopics / admin foundation

## Scope

- [ ] Decode DeleteRecords request
- [ ] Map to Iggy retention trim or document limitation
- [ ] Per-partition low watermark in response
- [ ] Test: delete records before offset X; Fetch cannot read deleted range

## Acceptance criteria

- [ ] Behavior documented if Iggy cannot trim arbitrarily (partial support OK with clear errors)

## References

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

Start with the Kafka gateway's DeleteRecords (21) request handling and the Iggy retention or explicit-delete support, using the DeleteTopics/admin foundation from #3546 as context. Verify per-partition low-watermark responses and the behavior of Fetch after records before offset X are removed; document clear errors where arbitrary trimming is unsupported.

Written by the indexing model from the issue text.