test: port 787/929 cross-module match carriers to corpus fixtures

787_xmod_variant_match.c -> r787_xmod_variant_no_default (run-exit 21),
  r787_xmod_variant_bind_default (42), r787_xmod_variant_shape_a (12)
  (pkg/errs module dirs), r787_xmod_variant_foreign_qualifier (staged
  error: w6c 'is not a variant of', w6c_ww rejects via the #95
  ambiguity guard — the carrier asserted reject polarity only)
929_match_4arm_cross_module_run.c -> r929_match_{4arm_shadowed_
  canonical,3arm_shadowed,5arm_shadowed,6arm_shadowed,4arm_mixed_kinds,
  4arm_shadowed_reverse} (run; single-file a+b packages, dropping the
  carrier's accidental doubled 'package b;' line)

929's byte-id upgrades from 994-delegated to per-fixture via
test-data-byteid; the asm-level distinct-CMPQ sentinel remains 728's
separate concern. All rows probed green on both stages.

test/wcc/data/r787_xmod_variant_shape_a/case.ww

@@ -0,0 +1,14 @@
+//ww:run-exit 12
+// migrated from test/wcc/787_xmod_variant_match.c: Shape A — a dotted body variant (errs.bad) keeps ITS OWN qualifier in case matching.
+package main;
+import pkg;
+import errs;
+fn classify(x: pkg.e) i32 = {
+	match (x) {
+	case errs.bad => return 1;
+	case pkg.local => return 2;
+	};
+};
+export fn main() i32 = {
+	return classify(pkg.mkbad()) * 10 + classify(pkg.mklocal());
+};

test/wcc/data/r787_xmod_variant_shape_a/errs/errs.ww

@@ -0,0 +1,3 @@
+// the foreign variant module for the r787 Shape A scenario.
+package errs;
+export type bad = !void;

test/wcc/data/r787_xmod_variant_shape_a/pkg/pkg.ww

@@ -0,0 +1,7 @@
+// union whose BODY holds a dotted cross-module variant (errs.bad | local).
+package pkg;
+import errs;
+export type local = !void;
+export type e = !(errs.bad | local);
+export fn mkbad() e = { let x: errs.bad; return x; };
+export fn mklocal() e = { let y: local; return y; };