Vectors: the malformed current-layout catalogs carry one entry and only its records

The older-layout reader refused the second current-layout entry on its own, so an unconditional fallback still refused the vector.
With one entry, only a store not in the older layout is refused for its malformed slot, and the fallback's condition is pinned.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

tests/vectors/document-variants-c0/generate.py

@@ -61,4 +61,6 @@ for name, entry in (
     ("current-catalog-bad-members", current[:6] + [5]),
     ("current-catalog-bad-compat", current[:5] + [[7]] + current[6:]),
 ):
-    write(name + ".msgpack", [header, [entry] + catalog[1:], records])
+    # One catalog entry and the records that use it: the older-layout reader takes an entry's slot 5 for a string, so a second
+    # current-layout entry would make it refuse for that reason, not for the malformed slot under test.
+    write(name + ".msgpack", [header, [entry], [record for record in records if record[1] == current[0]]])