s3api: document Checksum attribute as not yet populated

Checksum is accepted in validation (so clients requesting it don't get
a 400 error, matching AWS behavior for objects without checksums) but
SeaweedFS does not yet store S3 checksums. Add a comment explaining
this and noting where to populate it when checksum storage is added.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

weed/s3api/s3api_object_handlers_attributes.go

@@ -231,6 +231,11 @@ func (s3a *S3ApiServer) GetObjectAttributesHandler(w http.ResponseWriter, r *htt
 		resp.StorageClass = storageClass
 	}
 
+	// Checksum: accepted in validation so clients don't get a 400, but SeaweedFS
+	// does not yet store S3 checksums (CRC32, CRC32C, SHA1, SHA256), so
+	// resp.Checksum is intentionally left nil. When checksum storage is added,
+	// populate resp.Checksum here.
+
 	if _, ok := requestedAttrs["ObjectSize"]; ok {
 		var size int64
 		if entry.Attributes != nil {