A str field reached through a chained dot (o.i.s) emitted two loads
(ptr, len) and stored a stale CX as the cap — both stages, at any
non-zero chain depth. Emit the full header at the chained-dot leaf.
Review item #29.
Both stages move in one commit: one emission contract; splitting
would leave the byte-id gates red between the halves.
Hojun-Cho
f00775759d