drain(#357 post-merge): respect CommonMark 4-space-indent limit on fence detection

tools/pr-preservation/archive-pr.sh

@@ -523,15 +523,27 @@ for raw_line in content.split('\n'):
     #     This is how CommonMark lets you nest fences —
     #     a longer opener contains shorter fence-shaped
     #     lines as literal content.
-    stripped = raw_line.lstrip()
+    # Per CommonMark §4.5: opening fence permits up to 3
+    # *spaces* of indentation. 4+ spaces makes the line an
+    # indented-code-block, not a fence. Tabs are NOT space-
+    # equivalent here — a tab pushes column >= 4 = indented-
+    # code-block territory, and a tab-indented fence-shaped
+    # line is content, not a fence. Count literal leading
+    # spaces only (lstrip(' '), not bare lstrip() which would
+    # also consume tabs), enforce the <= 3 cap, AND reject any
+    # tab in the prefix.
+    leading_space_count = len(raw_line) - len(raw_line.lstrip(' '))
+    leading_chars = raw_line[:leading_space_count]
+    after_spaces = raw_line[leading_space_count:]
     marker = None
     marker_len = 0
-    if stripped.startswith('```'):
-        marker = '`'
-        marker_len = len(stripped) - len(stripped.lstrip('`'))
-    elif stripped.startswith('~~~'):
-        marker = '~'
-        marker_len = len(stripped) - len(stripped.lstrip('~'))
+    if leading_space_count <= 3 and '\t' not in leading_chars:
+        if after_spaces.startswith('```'):
+            marker = '`'
+            marker_len = len(after_spaces) - len(after_spaces.lstrip('`'))
+        elif after_spaces.startswith('~~~'):
+            marker = '~'
+            marker_len = len(after_spaces) - len(after_spaces.lstrip('~'))
     if marker is not None:
         if not in_fence:
             # Opening fence: record marker + length so the