Carry the machine-assigned port in a plan's contribution, so a provider serving from plans dials where the module listens (issue 774, research 072 row 34)

route-proxy served every route at the software's own port (gitea's 3000, published at 20000;
grafana's 3000 on another machine, published at 20012) and public sites answered 502. The plan's
contribution now carries the port the placement assigned, the number ${port:N} gives the module,
and a composition for plans refuses a port none assigned by module, node and port.

go/compose/render.go

@@ -58,6 +58,8 @@ func (c *composer) render(nodeName string, mode Mode) (Rendered, error) {
 	}
 	r := placed.Rendering
 	out.ForeseenPorts = placed.ForeseenPorts
+	// A plan carries its contributions' machine ports, refused where the placement assigned none (issue 774).
+	r.Planned = mode == Planning
 	r.Grants, r.Withheld, r.Unbound = grants, withheld, unbound
 	out.Withheld, out.Unbound = withheld, unbound
 	for _, g := range grantsForeseen {