docs(6209): test prose in the present tense — "used to", "previously", "before this" restated as the failure shape

Twenty-eight test files described the code they guard by what it did
before a fix. Each is now the property and the failure it prevents, in the
present tense: the backfill snapshot, the per-dial calibration query, the
rule scope, the lesson trigger filter, the subshell global, the trash owner
scope, the version readout fallback, and others. A docstring line that
narrated the backup suite's own history is removed.

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

tests/test_preference_write_path.py

@@ -160,7 +160,7 @@ async def test_update_preference_forwards_what_taught_the_change():
 
 def test_the_rule_door_still_asks_before_writing():
     """Half one of the asymmetry. If this fails, the gate was tidied away and
-    preferences are no longer the exception — they are just the same thing.
+    preferences are not the exception — they are just the same thing.
     """
     doc = _doc(MODULE, "create_rule").lower()
     asks = ("approve", "propose", "ask", "question")