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_system_usage_readout.py

@@ -1,7 +1,6 @@
-"""Rulings are counted where someone will read them (#4769).
+"""Rulings are counted where someone will read them.
 
-`system_usage_events` was written from milestone 444 step 4 and read by
-nothing. These pin the two readouts:
+`system_usage_events` is read in two places, and these pin both readouts:
 
 - PER SYSTEM: `usage_for_systems` — one GROUP BY, a zero shape for a System
   nothing touched — carried by the Systems list, the REST detail and MCP
@@ -10,7 +9,7 @@ nothing. These pin the two readouts:
   the areas named, and deliberately NO pull-through ratio.
 
 Integration for both aggregates, not mocked: a GROUP BY the database rejects
-is swallowed by the fail-open guard and reads as zero, which is #2663 exactly.
+is swallowed by the fail-open guard and reads as zero.
 """
 from unittest.mock import AsyncMock, patch