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_a6_concrete/case.ww

@@ -0,0 +1,9 @@
+//ww:run
+// migrated from test/wcc/839_xmod_nominal_typeeqast.c: A6 — concrete e.myerr returned into e.res (bare-vs-qualified nominal forward), both stages accept.
+package e;
+export type myerr = !i64;
+export type res = (i64 | myerr);
+package main;
+import e;
+fn wrap(x: e.myerr) e.res = { return x; };
+export fn main() i32 = { return 0; };