Check a delivery group's member beside its siblings' merges, judged by the group's controller (issue 867, ADR 0382)

A mesh-host change and a mesh-controller change that need each other could not both pass their own checks: each read the other at its base. The check now asks mesh-delivery for the group's open siblings into a base of the same name, reads each at its head merged onto its base, builds the judge from a controller sibling's merge in the isolated judge environment keyed on its inputs, and names the siblings in the verdict. With no answer from mesh-delivery the verdict says it could not learn them; a pull request with no sibling is checked as before. TestReplay867 replays the failure.

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

cmd/mesh-builder/main.go

@@ -337,7 +337,8 @@ 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, Validator: v.Validator}
+				Release: v.Release, JudgedBy: v.JudgedBy, BesideAt: v.BesideAt, Validator: v.Validator,
+				Siblings: siblingsOut(v.Siblings), SiblingsUnknown: v.SiblingsUnknown, JudgeFrom: v.JudgeFrom}
 			// 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,
@@ -467,7 +468,11 @@ func checkSpecOf(request link.BuildRequest) builder.CheckSpec {
 		Owner: c.Owner, Repo: c.Repo, Number: c.Number, Paths: c.Paths, Beside: map[string]builder.Beside{},
 		Modules: c.Modules, New: c.New, Manifests: c.Manifests, Judge: c.Judge, Base: c.Base,
 		Toolchain: builder.ToolchainOf(request.Held), Toolchains: builder.ToolchainsOf(request.Held),
-		Tests: c.Tests, StoreOnly: c.StoreOnly, Release: c.Release}
+		Tests: c.Tests, StoreOnly: c.StoreOnly, Release: c.Release, SiblingsUnknown: c.SiblingsUnknown}
+	for _, s := range c.Siblings {
+		spec.Siblings = append(spec.Siblings, builder.Sibling{Repository: s.Repository, Number: s.Number, Head: s.Head,
+			Base: s.Base, Dir: s.Dir})
+	}
 	for dir, b := range c.Beside {
 		spec.Beside[dir] = builder.Beside{Repository: b.Repository, Ref: b.Ref, Else: b.Else}
 	}
@@ -482,6 +487,16 @@ func checkSpecOf(request link.BuildRequest) builder.CheckSpec {
 	return spec
 }
 
+// siblingsOut is the siblings a check read as the outcome carries them (novox/hq ADR 0382 §4).
+func siblingsOut(read []builder.JudgedSibling) []link.CheckSibling {
+	var out []link.CheckSibling
+	for _, s := range read {
+		out = append(out, link.CheckSibling{Repository: s.Repository, Number: s.Number, Head: s.Head, Base: s.Base,
+			BaseCommit: s.BaseCommit})
+	}
+	return out
+}
+
 // judgeCache is the judge cache the build agent's manifest declares (MESH_JUDGE_CACHE, novox/hq ADR 0358 §6): a
 // directory mounted at the same path on the host, so a judge build's mounts name what the agent sees. Empty when the
 // agent declares none, and every judge is then built cold under its check's root.