Say which validator judged a release-branch check, and prove clause 5

ADR 0358 version 4 asks the verdict to say whether mesh-host's release-branch
validator or the vendored one judged, and checks that a release branch's own
pull request into its trunk is judged as any trunk pull request (issue 704).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

cmd/mesh-builder/main.go

@@ -331,7 +331,7 @@ func answer(ctx context.Context, publisher builder.Publisher, on, workspace stri
 			result.Check = &link.CheckOutcome{Verdict: v.Verdict, Summary: v.Summary, Report: v.Report,
 				Took: v.Took.Round(time.Second).String(), Gate: layerOf(v.Gate), RepoCheck: layerOf(v.Repo),
 				Head: v.Head, Trunk: v.Trunk, Beside: v.Beside, Built: checkBuiltOf(v.Built),
-				Release: v.Release, JudgedBy: v.JudgedBy, BesideAt: v.BesideAt}
+				Release: v.Release, JudgedBy: v.JudgedBy, BesideAt: v.BesideAt, Validator: v.Validator}
 			// What each of a group's heads was judged as (novox/hq ADR 0323 §6).
 			for _, m := range v.Members {
 				result.Check.Members = append(result.Check.Members, link.CheckedMember{Owner: m.Owner, Repo: m.Repo,