Port head-only of cstage cmd/wcc/check.c:1212-1213 to exprtype's
new N_SPREAD arm. The spread expression `xs...` carries the
operand's type — pass-through stamp, no structural synthesis.
A.6.2 step 5 of 8 (γ order). The smallest stamp arm in the
series; mirrors cstage's one-liner directly.
`make test-unit` green; full `make test` batched per option B.
Hojun-Cho
afd62a9187