fix: return checksums from CopyObject

[Vonng]

Summary

• add the five checksum fields supported by SILO plus ChecksumType to CopyObjectResponse
• populate CopyObjectResult from the committed destination ObjectInfo, including encrypted checksum metadata
• verify response values for every supported algorithm, default CRC64NVME, SSE-C, SSE-S3, compressed destinations, multipart-composite sources, and boundary sizes
• register the six exported response fields in the compatibility baseline

Why

SILO stored CopyObject checksums and exposed them through HEAD/GET, but the successful CopyObject XML contained only LastModified and ETag. AWS CopyObjectResult defines the checksum elements, and SDK callers expect them when the destination has a checksum.

This response-contract fix is deliberately separate from #66, which corrected the byte domain covered by the stored checksum.

Downstream

Active minio-go also drops these XML elements because its private copyObjectResult contains only ETag and LastModified. The client follow-up is tracked upstream as minio/minio-go#2294.

MCLI and SILO Console require no direct changes: MCLI ignores CopyObject UploadInfo, while Console only passes it through.

Verification

• response assertions across the complete Issue 63 API matrix
• focused race tests
• go test ./cmd -count=1
• go vet ./cmd
• golangci-lint v2.13.1: 0 issues
• regenerated compatibility baseline followed by make rebrand-guard

Fixes #68