One machine's unanswerable requirement does not remove it from the mesh

The pass that answers *what does this node offer* takes a failed resolution to
mean it learned nothing about that node. So refusing an unanswerable
requirement there made the machine disappear — and every other machine was then
told, wrongly, that the two of them shared no private network.

A wrong answer about a machine nobody asked about, caused by a fault on a
third. The lab found it: one module needing a licence that had not been added
yet made two unrelated machines look disconnected.

The second pass still refuses it, where the question is actually being asked.

internal/catalogue/licence_test.go

@@ -148,3 +148,40 @@ func TestALicenceDeliversWhatIsPublicAndWhatIsSealed(t *testing.T) {
 		t.Fatal("the key was written into the public file as well")
 	}
 }
+
+// One machine's unresolvable module must not remove it from the private network.
+//
+// The pass that answers *what does this node offer* takes a failed resolution to mean it learned
+// nothing — so refusing an unanswerable requirement there makes the machine disappear, and every
+// other machine is then told, wrongly, that the two of them share no network. A wrong answer about
+// a machine nobody asked about, caused by a fault on a third.
+func TestAnUnanswerableRequirementDoesNotRemoveAMachineFromTheMesh(t *testing.T) {
+	shelf := map[string]Manifest{
+		"assistant": aModelUser(),
+		"networking": {Module: "networking",
+			Provides: []Offer{{Name: "mesh-network", Scope: ScopeNode}}},
+	}
+	// The first pass: what does this machine offer? It is assigned something nothing answers.
+	got, err := Resolve(shelf, []string{"assistant", "networking"},
+		Node{Name: "laptop"}, World{Unchecked: true})
+	if err != nil {
+		t.Fatalf("a machine with one unanswerable requirement was lost entirely: %v", err)
+	}
+	var offers bool
+	for _, m := range got.Modules {
+		for _, o := range m.Offers() {
+			if o == "mesh-network" {
+				offers = true
+			}
+		}
+	}
+	if !offers {
+		t.Fatal("the machine's own network module was not seen, so it looks off the network")
+	}
+
+	// And the second pass, where the question is actually being asked, still refuses it.
+	if _, err := Resolve(shelf, []string{"assistant", "networking"},
+		Node{Name: "laptop"}, World{}); err == nil {
+		t.Fatal("the requirement nothing answers was accepted when it was actually asked")
+	}
+}