- Add EXACT_OFFSET and RESET_TO_OFFSET to OffsetType enum
- Add start_offset field to PartitionOffset for offset-based positioning
- Add base_offset and last_offset fields to PublishRecordResponse
- Add offset field to SubscribeRecordResponse
- Regenerate protobuf Go code
- Add comprehensive tests for proto serialization and backward compatibility
- All tests pass, ready for Phase 2 implementation