feat: support COPY of a single entry

merge_srt.py

@@ -87,7 +87,10 @@ class SRTMerger:
             source = int(parts[1])
             if source not in (1, 2):
                 return None
-            start, end = map(int, parts[2].split('-'))
+            if '-' in parts[2]:
+                start, end = map(int, parts[2].split('-'))
+            else:
+                start = end = int(parts[2])
             if start > end:
                 return None
         except (ValueError, AttributeError, TypeError):