docs(6209): seventeen more test files — lesson shape and round trip, rule scope/moments/kind/surfacing, drift, deadlock race, backup, inception, backfill, claims, supersession, ledgers, family shapes

Docstrings and comments state the property and its reason in the present
tense. The session-context ledger test no longer describes the decision it
"used to pin"; the supersession tests state demote-not-filter without the
quote; one-row measurements become the general shape with their figures.

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

tests/test_supersession_ranking.py

@@ -1,9 +1,7 @@
 """Demotion: superseded records rank behind their equals, and are never hidden.
 
-Step 3 of #278. The distinction these tests exist to protect is DEMOTE vs
-FILTER. The operator was explicit:
-
-    "the failure is pollution, not existence"
+The distinction these tests exist to protect is DEMOTE vs FILTER: the failure
+is pollution, not existence.
 
 Hiding a superseded record would turn every one of them into something you must
 already know exists in order to find, and would destroy "what did we think
@@ -131,11 +129,10 @@ async def test_an_empty_candidate_set_short_circuits():
 def test_the_penalty_is_sized_to_reorder_a_cluster_not_shuffle_within_it():
     """Sized against a measurement, not by feel.
 
-    Measured 2026-08-07 (#2486): the notes competing on a dev-log's own title
-    phrase sat within ~0.014 of each other, spanning 0.6506 down to 0.6120. A
-    penalty smaller than that spread would move a record within a tie without
-    changing which one wins — the failure #2486 already proved cannot be tuned
-    away, because the neighbours are not barely passing, they are tied.
+    Notes competing on a dev-log's own title phrase sit within ~0.014 of each
+    other (0.6506 down to 0.6120 on one measured cluster). A penalty smaller
+    than that spread moves a record within a tie without changing which one
+    wins — a failure that cannot be tuned away, because the neighbours are not barely passing, they are tied.
 
     The upper bound is the operator's constraint, not an optimisation: a penalty
     large enough to bury a superseded record outright is hiding by another name.