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>

internal/link/build.go

@@ -173,6 +173,13 @@ type CheckRequest struct {
 	// node-engine with its own validator — and each repository beside it is read at its branch of that name where it
 	// has one (CheckedOut.Else otherwise). Empty for every other base, checked as before it.
 	Release string `json:"release,omitempty"`
+	// Siblings are the other open pull requests of this one's delivery group the check reads beside it (novox/hq ADR
+	// 0382 §1): each repository found under Dir is read at the sibling's head merged onto the sibling's base, in
+	// place of the commit the mesh runs. SiblingsUnknown says mesh-delivery could not be asked, or answered without
+	// what a sibling is found by, and the check reads none. Both empty when mesh-delivery answered and named none:
+	// the request is then what it was before the rule.
+	Siblings        []CheckSibling `json:"siblings,omitempty"`
+	SiblingsUnknown string         `json:"siblings_unknown,omitempty"`
 	// Group names a delivery group whose members' heads are composed together with this one (novox/hq ADR
 	// 0239), and Members are those other heads, each cloned beside it and laid over the mesh in turn. The
 	// repository's own check is not run for a group: each member's pull request runs its own.
@@ -250,6 +257,19 @@ type CheckBuilt struct {
 	SameAs map[string]string `json:"same-as,omitempty"`
 }
 
+// CheckSibling is one other open pull request of a delivery group (novox/hq ADR 0382): in a request, the repository it
+// is in as owner/repo, the directory the check finds that repository under (Dir), its number, its whole head and its
+// base branch; in a verdict, what the check read of it — Head, Base and BaseCommit, whole: the base's head the sibling's
+// head was merged onto.
+type CheckSibling struct {
+	Repository string `json:"repository"`
+	Number     int    `json:"number"`
+	Head       string `json:"head"`
+	Base       string `json:"base"`
+	BaseCommit string `json:"base_commit,omitempty"`
+	Dir        string `json:"dir,omitempty"`
+}
+
 // GroupMember is one other head of a delivery group's composed check.
 type GroupMember struct {
 	Owner      string   `json:"owner"`
@@ -309,6 +329,12 @@ type CheckOutcome struct {
 	BesideAt map[string]string `json:"beside-at,omitempty"`
 	// Validator says which node-engine validator the judge of a release-branch check was built with (ADR 0358 §2).
 	Validator string `json:"validator,omitempty"`
+	// Siblings are the pull requests of its delivery group it read beside it, each as its head merged onto its base
+	// commit (ADR 0382 §4), SiblingsUnknown why it could not learn them, and JudgeFrom where the judge came from when
+	// the group made it (§3), such as "sibling novox/mesh-controller#346 merge 0123456789ab".
+	Siblings        []CheckSibling `json:"siblings,omitempty"`
+	SiblingsUnknown string         `json:"siblings_unknown,omitempty"`
+	JudgeFrom       string         `json:"judge_from,omitempty"`
 }
 
 // SourceOnSeat names a repository by the seat whose holder serves it and its path there.

internal/link/events.go

@@ -301,6 +301,13 @@ type Checked struct {
 	BesideAt map[string]string `json:"beside-at,omitempty"`
 	// Validator says which node-engine validator the judge of a release-branch check was built with (ADR 0358 §2).
 	Validator string `json:"validator,omitempty"`
+	// Siblings are the other pull requests of the delivery group the check read beside it (novox/hq ADR 0382 §4), each
+	// with its head and the base commit it was merged onto; SiblingsUnknown is why it could not learn them, set only
+	// then; JudgeFrom says where the judge came from when the group made it. The forge's holder keeps Siblings per
+	// open pull request (§5). All omitted for a check that read none.
+	Siblings        []CheckSibling `json:"siblings,omitempty"`
+	SiblingsUnknown string         `json:"siblings_unknown,omitempty"`
+	JudgeFrom       string         `json:"judge_from,omitempty"`
 	// Built are the builds the check made for its merge, each with its reuse key and digests, or why not
 	// (novox/hq ADR 0350 §1): what the walk of the merge takes instead of building.
 	Built []CheckBuilt `json:"built,omitempty"`