wcc: enums compare nominally; folded members keep their enum stamp

Two composing defects made a two-enum union mis-tag in wwstage
(live cs!=ww: `let e: (color|shape) = shape.BALL` stored tag 0 —
the color arm — while cstage stored 1). type_eq/typeeq had no
TY_ENUM arm, so ANY two enums fell into the primitive default and
compared equal; enums are nominal (harec: an enum IS its alias
type) and now compare by node identity only. Underneath, the
wwstage post-order revisit re-stamped the constant-folded enum
member (an N_INTLIT) as untyped_int, clobbering the enum stamp the
N_DOT fold applied, so the widen matcher fell to its first-variant
fallback -- the #59.9 N_BIN guard now twins on N_INTLIT (cstage
cexpr is single-pass and never clobbered).

cmd/wcc/type.c

@@ -275,6 +275,15 @@ type_eq(Type *a, Type *b)
 	}
 	case TY_NAMED:
 		return a == b;	/* nominally equal only when same node */
+	case TY_ENUM:
+		/* Enums are nominal (harec: an enum IS its alias type):
+		 * distinct decls are distinct types even with identical
+		 * storage/members. Falling into the primitive default made
+		 * ANY two enums equal — the wwstage variant matcher picked
+		 * the first enum variant of a union for a value of the
+		 * OTHER enum (live cs!=ww, wrong tag at runtime). Same-decl
+		 * chased ends hit the a == b fast path above. */
+		return 0;
 	case TY_TUPLE: {
 		Tparam *pa = a->params, *pb = b->params;
 		while (pa && pb) {