fix: compute multipart part checksums server-side

Accept checksum-enabled UploadPart and UploadPartCopy requests when clients omit optional per-part checksum headers. Compute over the logical plaintext stream, persist the result, and return it from CopyPartResult while preserving client checksum validation.\n\nRefs #46; follow-ups #63 and #64 remain out of scope.

cmd/api-response.go

@@ -387,6 +387,12 @@ type CopyObjectPartResponse struct {
 	XMLName      xml.Name `xml:"http://s3.amazonaws.com/doc/2006-03-01/ CopyPartResult" json:"-"`
 	LastModified string   // time string of format "2006-01-02T15:04:05.000Z"
 	ETag         string   // md5sum of the copied object part.
+
+	ChecksumCRC32     string `xml:",omitempty"`
+	ChecksumCRC32C    string `xml:",omitempty"`
+	ChecksumSHA1      string `xml:",omitempty"`
+	ChecksumSHA256    string `xml:",omitempty"`
+	ChecksumCRC64NVME string `xml:",omitempty"`
 }
 
 // Initiator inherit from Owner struct, fields are same
@@ -771,11 +777,16 @@ func generateCopyObjectResponse(etag string, lastModified time.Time) CopyObjectR
 	}
 }
 
-// generates CopyObjectPartResponse from etag and lastModified time.
-func generateCopyObjectPartResponse(etag string, lastModified time.Time) CopyObjectPartResponse {
+// generates CopyObjectPartResponse from the uploaded part information.
+func generateCopyObjectPartResponse(partInfo PartInfo) CopyObjectPartResponse {
 	return CopyObjectPartResponse{
-		ETag:         "\"" + etag + "\"",
-		LastModified: amztime.ISO8601Format(lastModified.UTC()),
+		ETag:              "\"" + partInfo.ETag + "\"",
+		LastModified:      amztime.ISO8601Format(partInfo.LastModified.UTC()),
+		ChecksumCRC32:     partInfo.ChecksumCRC32,
+		ChecksumCRC32C:    partInfo.ChecksumCRC32C,
+		ChecksumSHA1:      partInfo.ChecksumSHA1,
+		ChecksumSHA256:    partInfo.ChecksumSHA256,
+		ChecksumCRC64NVME: partInfo.ChecksumCRC64NVME,
 	}
 }