Refuse an apply that would remove a resource of a module a want still places here, so a composition that lost what runs can never remove the bus server or a held module again (issue 774)

internal/agent/agent.go

@@ -1042,6 +1042,20 @@ func (a *Agent) Everything() []delivery.Composed {
 	return a.others("\x00")
 }
 
+// Placed is the modules a want still places on the node (present and not removed), held ones included, but except:
+// what no apply but the module's own may remove (apply.Scope.Placed, issue 774). Called without a.mu held.
+func (a *Agent) Placed(except string) map[string]bool {
+	a.mu.Lock()
+	defer a.mu.Unlock()
+	out := map[string]bool{}
+	for m, w := range a.wants {
+		if m != NodeModule && m != except && w.Present && !w.Want.Removed {
+			out[m] = true
+		}
+	}
+	return out
+}
+
 // HoldsNode says whether the agent runs the node's own composition, so the node's parts are its and not the kept
 // declaration's.
 func (a *Agent) HoldsNode() bool {