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_bind_default/case.ww

@@ -0,0 +1,17 @@
+//ww:run-exit 42
+// migrated from test/wcc/787_xmod_variant_match.c: bare arm + bound `case let o: pkg.b` + default over an imported 3-variant union.
+package main;
+import pkg;
+fn classify(e: pkg.u2) i32 = {
+	match (e) {
+	case pkg.a => return 1;
+	case let o: pkg.b => return 2;
+	case => return 9;
+	};
+};
+export fn main() i32 = {
+	if (classify(pkg.mka()) != 1) { return 11; };
+	if (classify(pkg.mkb()) != 2) { return 12; };
+	if (classify(pkg.mkc()) != 9) { return 13; };
+	return 42;
+};

test/wcc/data/r787_xmod_variant_bind_default/pkg/pkg.ww

@@ -0,0 +1,9 @@
+// exports the 3-variant union for the r787 bind_and_default scenario.
+package pkg;
+export type a = !void;
+export type b = !void;
+export type c = !void;
+export type u2 = !(a | b | c);
+export fn mka() u2 = { let x: a; return x; };
+export fn mkb() u2 = { let y: b; return y; };
+export fn mkc() u2 = { let z: c; return z; };