The Warren that owns this member, or null when unaffiliated OR
when the previous Warren has destructed (R2.3 self-heal). Reads
always go through here — never a declared seed path.
Back-ref setter. Normal callers don't invoke this directly — it is
wired by Warren.addMember / removeMember (the relationship's
sole owners). Public so the symmetric Warren-side helpers can reach
it.
Public shape provided by WarrenMemberMixin.