fix: ruff UP031 — percent formatting in the new naming test (4392)

Run 7456's only failure. `"comic2_%02d.jpg" % i` is an f-string here; UP is
enabled repo-wide and I wrote the one construct it forbids.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

tests/test_post_naming.py

@@ -294,7 +294,7 @@ def test_a_name_reused_across_many_posts_is_still_caught():
     """The other half of the same property — the cap has to keep working once
     the unit changes. Measured habits: tamadaheijun's `comic2` spans 8 posts,
     conto's `seth2` 5."""
-    counts = token_frequencies([["comic2_%02d.jpg" % i] for i in range(8)])
+    counts = token_frequencies([[f"comic2_{i:02d}.jpg"] for i in range(8)])
 
     assert counts["comic2"] == 8
     assert shared_identity({"comic2"}, {"comic2"}, counts) == (0.0, None)