docs(claude.md): rule 16 refresh — generic parse helpers + fixture cruft

Rule 16 was written before PR 1 + PR 2a landed the generic parse_required
/ parse_optional helpers with Option<&str> signatures. The old wording
"Helper shape: fn parse_X(opt: &Option<String>)" is now actively wrong;
the canonical shape is calling the generic helpers directly with
.as_deref() (proto) or Some(s.as_str()) (text path).

Also adds the fixture-cruft caveat from PR #559: when the new strict
decoder rejects an existing test fixture, verify against
tws-api/samples/ before broadening FromStr. PR #559 caught
right: "?" as a VB-sample display fallback, not real TWS wire.