# POST3-005: XML response builders and extractors

**Status:** Done
**Priority:** P1
**Blocked by:** POST3-004

## Description

Implement S3-compatible XML response serialization and request query parameter extraction.

## Acceptance Criteria

- [ ] `s3/responses.rs`:
  - `list_buckets_xml(buckets)` — ListAllMyBucketsResult with Owner
  - `list_objects_v2_xml(bucket, result, max_keys)` — ListBucketResult with Contents
  - `error_xml(code, message, resource)` — S3 Error response
- [ ] `s3/extractors.rs`:
  - `ListObjectsQuery` — list-type, prefix, max-keys, continuation-token, start-after, delimiter
- [ ] XML output matches S3 format (xmlns, element names, date format ISO 8601)
- [ ] All responses include `x-amz-request-id` header (UUID)