Non-descriptor Constants set attributes accept bare-string assignment; substring 'in' silently corrupts parsing #241

The four cached-union attributes (titles, prefixes, suffix_acronyms, suffix_not_acronyms) are guarded by _CachedUnionMember.__set__, which rejects non-SetManager assignment with a clear TypeError. The other five set-backed attributes — first_name_titles, conjunctions, bound_first_names, non_first_name_prefixes, suffix_acronyms_ambiguous — are plain instance attributes (config/init.py, assigned in Constants.__init__), so this is accepted silently:

from nameparser import HumanName
from nameparser.config import Constants

c = Constants()
c.conjunctions = 'and'      # attribute is now a plain str
HumanName('John An Smith', constants=c)
# first='John An Smith', middle='', last=''   (default: John / An / Smith)

Root cause: membership checks like piece.lower() in self.C.conjunctions become substring tests on a str ('an' in 'and' is True), so parsing silently degrades with no traceback pointing at the bad assignment.

The constructor path is guarded since #238 (Constants(conjunctions='and') raises). This is the residual assignment gap. Distinct from #239, which is about hn.C assignment on HumanName, not attribute assignment on Constants.

Fix: guard these five like the cached-union four — a lighter shared descriptor enforcing isinstance(value, SetManager) (without the _pst invalidation), reusing the existing error-message style.

Found by review of PR #240 — pre-existing, unrelated to that change.