fix(02-03): mask collection_key and checkpoint in JSON diffs

Album sync payloads hash the collection and stamp a checkpoint that
change across seed runs and must not fail PHP self-replay.

Co-authored-by: Cursor <cursoragent@cursor.com>

tide/normalize.go

@@ -55,6 +55,16 @@ func maskLeaf(path string, val any, step Step, diffs *[]Diff) any {
 	if key == "slug" || disabledPath(step, path, key) {
 		return val
 	}
+	if key == "collection_key" {
+		if val == nil {
+			return nil
+		}
+		if _, ok := val.(string); !ok {
+			*diffs = append(*diffs, Diff{Path: path, Expected: "string collection_key", Actual: formatValue(val)})
+			return val
+		}
+		return maskID
+	}
 	if isDateKey(key) {
 		return maskDate(path, val, diffs)
 	}
@@ -97,7 +107,7 @@ func maskIDValue(path string, val any, diffs *[]Diff) any {
 }
 
 func isDateKey(key string) bool {
-	return strings.HasSuffix(key, "_at")
+	return strings.HasSuffix(key, "_at") || key == "checkpoint"
 }
 
 func isIDKey(key string) bool {