feat(14-03): tide keeps binary upstream bodies and compares every *_url upload by shape

- A response body that is not valid UTF-8 (a cover image) is written as a
  YAML !!binary scalar, never masked and replayed byte for byte
- The upload URL normalizer covers every key ending in _url (cover_url),
  not only url and thumb_url
- README and parity-testing docs updated

modules/tide/normalize.go

@@ -73,7 +73,7 @@ func maskLeaf(path string, val any, step Step, opts maskOptions, diffs *[]Diff)
 	if key == "slug" || disabledPath(step, path, key) {
 		return val
 	}
-	if key == "url" || key == "thumb_url" {
+	if isUploadURLKey(key) {
 		if s, ok := val.(string); ok {
 			return maskUploadURL(path, s, opts.prefix(), diffs)
 		}
@@ -101,6 +101,14 @@ func maskLeaf(path string, val any, step Step, opts maskOptions, diffs *[]Diff)
 	return val
 }
 
+// isUploadURLKey reports the keys whose string values are compared as
+// uploaded-file URLs: url and every key ending in _url (thumb_url,
+// cover_url). A value outside the uploads prefix that does not look like an
+// upload is compared as it is.
+func isUploadURLKey(key string) bool {
+	return key == "url" || strings.HasSuffix(key, "_url")
+}
+
 func maskDate(path string, val any, diffs *[]Diff) any {
 	if val == nil {
 		return nil