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).

test/wcc/data/enum_union_tag/case.ww

@@ -0,0 +1,20 @@
+//ww:run-exit 0
+// Two enums in one union: variant tags must resolve nominally. The
+// enum-blind typeeq matched ANY enum pair (wwstage widened shape.BALL
+// into the color variant, tag 0), and the post-order revisit clobbered
+// the folded member's enum stamp to untyped_int (first-variant
+// fallback). Both orderings pinned.
+package main;
+type color = enum { RED, GREEN };
+type shape = enum { BOX, BALL };
+type either = (color | shape);
+type either2 = (shape | color);
+export fn main() i32 = {
+	let e: either = shape.BALL;
+	if (!(e is shape)) { return 1; };
+	let f: either2 = color.RED;
+	if (!(f is color)) { return 2; };
+	let g: either = color.GREEN;
+	if (!(g is color)) { return 3; };
+	return 0;
+};