Only a thing built and never run is unassignable

The check read "declares no resources" as "runs nowhere", and those are not
the same. The private network declares no resources either — the control plane
computes them when it composes a machine's declaration — and it is assigned to
every machine that has to reach another one. Refusing it stopped a four-machine
bed at its first assignment.

The signal is narrower: it builds an artifact and places nothing. Made a
function of its own, because a judgement with a wrong answer this expensive
should be testable without a database — nothing guarded it, which is how it
shipped.

internal/inventory/catalogue.go

@@ -906,7 +906,7 @@ func (i *Inventory) runsSomewhere(ctx context.Context, module string) error {
 		// fault and refusing the assignment here would report it as the wrong one.
 		return nil
 	}
-	if m.Computed != "" || len(m.Resources) > 0 {
+	if !IsBuildInput(m) {
 		return nil
 	}
 	return fmt.Errorf(
@@ -914,3 +914,18 @@ func (i *Inventory) runsSomewhere(ctx context.Context, module string) error {
 			"builds and other modules are built on top of, not something a machine runs — "+
 			"`module list` shows what it produces", module)
 }
+
+// IsBuildInput reports whether a module exists to be built and never to be run.
+//
+// **The signal is that it builds something and places nothing** — not merely that it declares no
+// resources. Those are different, and confusing them refused a module the mesh itself ships: the
+// private network declares no resources either, because the control plane computes them when it
+// composes a machine's declaration, and it is assigned to every machine that has to reach another
+// one. Refusing it stopped a four-machine bed dead.
+//
+// Its own function because it is a judgement rather than a lookup, and a judgement with a wrong
+// answer this expensive should be testable without a database.
+func IsBuildInput(m catalogue.Manifest) bool {
+	builds := m.Build != nil && len(m.Build.Artifacts) > 0
+	return builds && m.Computed == "" && len(m.Resources) == 0
+}