Judge every pull request against the mesh that runs, before it merges (hq ADR 0237, to-be 45 §9)

Every check the mesh had ran after a merge, on a machine: a manifest the node-engine refused
(236), an identity a real machine's name made too long (263). merge-gate raises the mesh as the
facts snapshot says it is and the mesh with the change, each in a throwaway store through the
controller's own records, composes every machine twice and validates it with the node-engine's
validator, and fails what the change breaks, naming the machine's roles and the module - plus a
manifest the judging controller cannot read, a consumer left out of its grant, a module removed
while a machine runs it, a new module the node-engine would refuse; it warns on a wide rebuild.

The forge's new head of a pull request becomes a check the controller asks of the build seat:
the head and, beside it, the controller the mesh runs, the catalogue, the host and the lab; a
throwaway store and bus of the versions the mesh runs; the repository's merge-check.sh in the
mesh's Go toolchain with no container runtime socket; then mesh-lab's replays. The verdict is
said as checked, an error never a pass, and nothing is recorded or registered.

cmd/mesh-builder/main.go

@@ -237,7 +237,21 @@ func answer(ctx context.Context, publisher builder.Publisher, on, workspace stri
 
 	npmrc, err := packagesFrom()
 	var built builder.Result
-	if err == nil {
+	if request.Check != nil {
+		// **A pull request's merge check, not a build** (novox/hq to-be 45 §9): nothing is built,
+		// published or registered; the verdict is the outcome.
+		result.Checked = request.Check
+		registry := ""
+		if r, ok := publisher.(builder.Registry); ok {
+			registry = r.Address
+		}
+		var v builder.CheckVerdict
+		v, err = builder.Check(building, builder.Command, checkSpecOf(request), workspace, registry, forgeFrom(), say)
+		if err == nil {
+			result.Check = &link.CheckOutcome{Verdict: v.Verdict, Summary: v.Summary, Report: v.Report,
+				Took: v.Took.Round(time.Second).String()}
+		}
+	} else if err == nil {
 		// The package-registry credential is a build input, so it is resolved before the clone: a
 		// build that could not have resolved its dependencies is refused in front of the reason, not
 		// after a clone that then fails at npm ci.
@@ -263,6 +277,8 @@ func answer(ctx context.Context, publisher builder.Publisher, on, workspace stri
 		// builder that is not running, and those want completely different responses.
 		result.Failed = err.Error()
 		say("failed", err.Error())
+	} else if request.Check != nil {
+		say("checked", result.Check.Verdict+": "+result.Check.Summary)
 	} else {
 		manifest, marshalErr := json.Marshal(built.Manifest)
 		if marshalErr != nil {
@@ -312,6 +328,18 @@ func answer(ctx context.Context, publisher builder.Publisher, on, workspace stri
 	}
 }
 
+// checkSpecOf is a check request as the builder runs it.
+func checkSpecOf(request link.BuildRequest) builder.CheckSpec {
+	c := request.Check
+	spec := builder.CheckSpec{ID: request.ID, Repository: request.Repository, Ref: request.Ref,
+		Owner: c.Owner, Repo: c.Repo, Number: c.Number, Paths: c.Paths, Beside: map[string]builder.Beside{},
+		Toolchain: builder.ToolchainOf(request.Held)}
+	for dir, b := range c.Beside {
+		spec.Beside[dir] = builder.Beside{Repository: b.Repository, Ref: b.Ref}
+	}
+	return spec
+}
+
 // packagesFrom is where a build resolves the mesh's own published packages — the SDK above all
 // (novox/hq ADR 0076, issue 053).
 //