tools (volt): claim coverage 2 of 9 -> 8 of 9, and the sweep stops tripping the census

Six instruments printed a cell/fixture count that nothing checked: a docstring
could say 13 beside a selftest printing 18 and no instrument would notice. Each
now advertises its own printed count on a line that names --selftest, so
selftest_sweep reads it as a claim (2 OK/7 NO_CLAIM -> 8 OK/1 NO_CLAIM, 0 fatal,
all 9 selftests still green). doc_census_check.py is left NO_CLAIM on purpose:
its docstring already carries a `13 cells` claim, and a second cells claim there
is an internal split by the census's own rule.

selftest_sweep.py's own docstring quoted a fixture line ("selftest: 7 cells ...")
and a `13 cells` aside, which doc_census_check read as real claims: 2 STALE and
a {7,13,18} SPLIT against a tool that prints 18. Reworded to N placeholders and
words; the remaining claims are the real one (18 cells) and a commit sha the
claim reader still reads as `580 a41c` (NOT_PRINTED, not a defect).

tools/append_only_owners.py

@@ -43,7 +43,7 @@ can see whether the rule is being bent over status metadata or over prose:
   TIMED_CLAIM (carries a clock or ISO timestamp) / SHORT (<40 chars) / PROSE / EMPTY.
 
 USAGE
-  python3 append_only_owners.py AUDIT.json [--json OUT] [--selftest]
+  python3 append_only_owners.py AUDIT.json [--json OUT] [--selftest]   # 15 cells
     AUDIT.json is a --json output of tools/append_only_audit.py at the same rev.
 """
 import argparse