test: port 839_xmod_nominal_typeeqast to corpus fixtures

839_xmod_nominal_typeeqast.c -> r839_xmod_nominal_match (run-exit 42),
  r839_xmod_a6_concrete (run), r839_xmod_incompat_concrete (error
  'not assignable'), r839_xmod_distinct_alias (error 'ambiguous
  without nominal layout').

r839_xmod_nominal_match enters DATABYTEID_DIVERGED: the carrier waived
byte-id for the pre-existing cross-module tagged-return spill
divergence; the DIVERGED row makes it loud-on-fix instead. The
typeeqast-layer discriminator itself stays with the lib byteid gate
(test/byteid/libbyteid_test.ww), as the carrier documented.

test/wcc/data/r839_xmod_nominal_match/case.ww

@@ -0,0 +1,22 @@
+//ww:run-exit 42
+// migrated from test/wcc/839_xmod_nominal_typeeqast.c: bare-spelled union variants matched via QUALIFIED case patterns (`case let x: e.myerr`) — cross-module nominal cover.
+package e;
+export type myerr = !i64;
+export type res = (i64 | myerr);
+export fn ok(v: i64) res = { return v; };
+export fn bad() res = { return (-7: myerr); };
+package main;
+import e;
+fn unwrap(r: e.res) i64 = {
+	match (r) {
+	case let n: i64 => return n;
+	case let x: e.myerr => return -1;
+	};
+};
+export fn main() i32 = {
+	let a = unwrap(e.ok(40));
+	let b = unwrap(e.bad());
+	if (a != 40) { return 11; };
+	if (b != -1) { return 12; };
+	return 42;
+};