`arr[i] = mk()` where mk returns an in-cap (<=24B) struct/tuple/array
left the result in the #4 cgreturn registers (AX/DX/CX), but the
N_ASSIGN-into-N_INDEX path had no arm for an N_CALL rhs, so it fell to
the scalar store tail: only member 0 was written and the index scale
clobbered CX. Both stages emitted byte-identical wrong code (the
documented-but-silent #31-G gap), so the byte-id gate was blind to it.
Add an in-cap N_CALL-rhs arm: materialise the return into a frame scratch
first (keeping the CALL at the frame's natural 16B alignment), resolve
&arr[i], then word-copy the full eightbyte count + sub-8 tail -- mirroring
the #4 receive shape and the #270-1b copy. The eightbyte count derives
from the element size in the type table. Over-cap returns (#234),
non-call rhs (#270-1b) and tuple literals (#121) are unaffected; the
sibling field/deref shapes stay loud (#24).
Surfaced by the codegen miscompile hunt (finding C2c). Pinned by
test/lang/idx_aggret_recv_test.ww (10 value-asserting rows: 2/3-eightbyte
structs+tuples, array elem, sub-8 tail, const/runtime index, all four
base shapes; reddens on revert).
Hojun-Cho
c83a3403a4