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_backfill_reads_current_text.py

@@ -1,12 +1,12 @@
 """A backfill repairs to current truth; it does not replay a snapshot.
 
-The bug these pin: every backfill used to SELECT the text alongside the id and
-then iterate that snapshot, sleeping between records. A run over a real corpus
-takes minutes, so anything edited inside that window was re-embedded by its own
-write path and then OVERWRITTEN with the pre-edit text the scan had captured —
-and stamped at the current chunker version, so the next boot considered it
-current and never repaired it. The record stayed findable by what it used to
-say, silently, until someone happened to edit it again.
+The failure these pin: a backfill that SELECTs the text alongside the id and
+then iterates that snapshot, sleeping between records. A run over a real corpus
+takes minutes, so anything edited inside that window is re-embedded by its own
+write path and then OVERWRITTEN with the pre-edit text the scan captured — and
+stamped at the current chunker version, so the next boot considers it current
+and never repairs it. The record stays findable by what it used to say,
+silently, until someone happens to edit it again.
 
 That window is exactly "forward work continues while a long backfill runs",
 which is the normal condition, not an unlucky one.