The artifact store's seat is one per mesh, and the test says so from the catalogue

Review of the registry work found the seat node-scoped: a second `distribution` on another
machine resolved cleanly there, and only afterwards did the mesh notice `artifact-store`
offered by two nodes, with every consumer elsewhere refusing to choose. A node-scoped
requirement with one candidate installs that candidate, so anything that wanted the store
beside it would have raised a fresh, empty store on the wrong machine first.

The claim is mesh-scoped in mesh-catalog now; this holds the catalogue's manifest to it —
a second store anywhere is refused by name, where it is assigned.

internal/catalogue/artifact_store_seat_test.go

@@ -0,0 +1,38 @@
+package catalogue
+
+import (
+	"strings"
+	"testing"
+)
+
+// The artifact store's seat is one per mesh, read from the catalogue beside this checkout.
+//
+// **A second store anywhere is refused by name, not discovered as a consumer failure.** The seat
+// was node-scoped, so a second `distribution` on another machine resolved cleanly there — and a
+// node-scoped requirement with one candidate installs that candidate on the node, so anything that
+// required the store's presence beside it would have raised a fresh, empty store on the wrong
+// machine. Only afterwards did the mesh notice: `artifact-store` offered by two nodes, and every
+// consumer elsewhere refusing to choose. The claim says it first, where the second store is
+// assigned.
+func TestASecondArtifactStoreAnywhereIsRefusedByName(t *testing.T) {
+	store := catalogueManifest(t, "distribution")
+
+	// The first store resolves as it always has.
+	if _, err := Resolve(shelf(store), []string{"distribution"}, workstation(), World{}); err != nil {
+		t.Fatalf("the store alone does not resolve: %v", err)
+	}
+
+	// A second one, on any other machine, is refused — and the refusal names the seat.
+	elsewhere := World{Held: []Held{{Claim: "the-artifact-store", Scope: ScopeMesh,
+		Node: "anchor", Module: "distribution"}}}
+	other := workstation()
+	other.Name = "laptop"
+	_, err := Resolve(shelf(store), []string{"distribution"}, other, elsewhere)
+	if err == nil {
+		t.Fatal("a second store was accepted on another machine; it would offer artifact-store a " +
+			"second time and every consumer elsewhere would refuse to choose")
+	}
+	if !strings.Contains(err.Error(), "the-artifact-store") || !strings.Contains(err.Error(), "one per mesh") {
+		t.Fatalf("refused without naming the seat: %v", err)
+	}
+}