Point five stale S2 references at what exists

[derek73]

Follow-up to #716 (#682). Five references the S2 split's reviews found were already stale before the split. Comments only; no behavior change.

• _pieces.py, test_group.py, cases.py credited "rules/S2's initial veto", and test_policy.py "rules/S2's numeral fork". No rule's statement holds either clause, before or after the split.
  • The comma case is rules/C1's one-character suffix word, read by what stands before it, and the _pieces docstring now cites that.
  • The others now name the predicates: _vocab.is_initial and _vocab.is_trailing_numeral_suffix.
• cases.py's "row item 5 of decisions/S4" pointed at a numbered list in a gitignored spec. It now names the decisions/S4 entry it meant: 2026-09-15, THE COUNT REACHES THE LISTED SET TOO, which names Royce, Ed.

Not done here, on purpose. The trailing roman-numeral reading in a name with no comma is stated by no rule; C1 states only the comma case. Measured on master:

• John Smith V reads suffix 'V' and reports.
• John Smith V. and John J. V read family 'V.' and 'V'.
• John Smith v reads suffix with no report.
• John V reads suffix and loses its family.

Stating that as a rule means deciding those edges, so it is left to a separate design question rather than written down as found.

Tests: the affected modules pass (7,767), and ruff is clean.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.03%. Comparing base (c325bed) to head (5f504fd).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #717   +/-   ##
=======================================
  Coverage   99.03%   99.03%           
=======================================
  Files          46       46           
  Lines        4557     4557           
=======================================
  Hits         4513     4513           
  Misses         44       44           

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