Port head-only of cstage cmd/wcc/check.c:1230-1236 to exprtype's
new N_RECV arm. Peel alias on the channel base; `chan T` → T.
N_TCHAN element lives in .lhs (cstage ww.h:279, ww typeeqast
L585).
One documented divergence: cstage L1234 errors on a non-chan
base; wwstage returns nil under the lenient-on-miss policy
(scruttype L656 / A.6.1.5b N_DOT precedent).
A.6.2 step 4 of 8 (γ order). Same shape as A.6.2.0a N_SLICE
scaffold — precedent-consistent.
`make test-unit` green; full `make test` batched per option B.
Hojun-Cho
48c758bbe7