runww: //ww:error arm asserts both stages reject (dual-stage)

A rejected program emits no .s, so the test-lang-byteid (T2) gate cannot
cover wwstage-reject -- yet the retired C twins asserted that BOTH stages
reject with the same diagnostic. runww's //ww:error arm was cstage-only,
so migrating reject rows onto it would silently drop the wwstage-reject
coverage the C twins carried.

Run w6c_ww (wwstage) alongside w6c (cstage) on each //ww:error case and
require both to fail with the shared diagnostic body present. The body is
identical across stages; only cstage's leading prefix differs, so the
substring matches the body alone (no file:line). An ERROR row now reports
PASS dual / FAIL cstage / FAIL wwstage. w6c_ww resolves off the same $BIN
as the C twins -- no new harness threading.

Two pilot reject cases (runww_dup_main_reject, runww_dup_type_reject)
exercise the dual-stage path; the wwstage leg is proven non-vacuous (a
cstage-rejects/wwstage-accepts case reports FAIL wwstage).

Prerequisite for migrating fold-3 reject rows onto runww.

test/wcc/data/runww_dup_main_reject/case.ww

@@ -0,0 +1,8 @@
+//ww:error "duplicate fn main"
+// Pilot for #7a: two `fn main` definitions must be LOUD-rejected by BOTH
+// stages. The substring is the shared diagnostic body (no file:line prefix,
+// which differs cstage vs wwstage) — the #20 non-vacuity anchor and the
+// dual-stage (w6c AND w6c_ww) reject-coverage proof.
+package main;
+fn main() i32 = { return 0; };
+fn main() i32 = { return 1; };

test/wcc/data/runww_dup_type_reject/case.ww

@@ -0,0 +1,9 @@
+//ww:error "duplicate type t"
+// Pilot for #7a: a redefined type alias must be LOUD-rejected by BOTH stages.
+// The substring is the shared diagnostic body (no file:line prefix, which
+// differs cstage vs wwstage) — the #20 non-vacuity anchor and the dual-stage
+// (w6c AND w6c_ww) reject-coverage proof.
+package main;
+type t = i32;
+type t = i64;
+fn main() i32 = { return 0; };