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/r929_match_3arm_shadowed/case.ww

@@ -0,0 +1,26 @@
+//ww:run
+// migrated from test/wcc/929_match_4arm_cross_module_run.c: 3-arm boundary — arm 2 must reach its body under the shadowed callee.
+package a;
+export type more = void;
+export type done = void;
+export fn next(k: i32) (rune | done | more) = {
+    if (k == 0) { return 0x42u32: rune; };
+    if (k == 1) { let v: done; return v; };
+    let v: more; return v;
+};
+package b;
+import a;
+type done = void;
+fn next(k: i32) i32 = {
+    match (a.next(k)) {
+    case let r: rune    => return 1i32 + (r: i32);
+    case let dn: a.done => return 2;
+    case let m:  a.more => return 3;
+    };
+};
+export fn main() i32 = {
+    if (next(0) != 67) { return 11; };
+    if (next(1) != 2)  { return 12; };
+    if (next(2) != 3)  { return 13; };
+    return 0;
+};