Let the Reserve button pick another address of the lease's subnet

The Reserve dialog fills in the lease's address but lets it be changed;
POST /leases/{ip}/reserve takes an optional ip, steward lease reserve
takes --ip. The address must be in the lease's subnet.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

cmd/steward/leases.go

@@ -126,7 +126,7 @@ gets a NAK when it next renews, and then asks for a new address.`,
 	}
 	del.Flags().BoolVarP(&yes, "yes", "y", false, "don't ask")
 	cmd.AddCommand(del)
-	var hostname, comment, by string
+	var hostname, comment, by, ip string
 	reserve := &cobra.Command{
 		Use:   "reserve IP",
 		Short: "Make a lease's address its client's reservation",
@@ -136,7 +136,10 @@ then on. The host name the client sent becomes the reservation's, unless
 
 The reservation knows the client by its MAC address and, if it sent one,
 its client ID (option 61): either one gets it the address. --by mac or
---by client-id uses just one.`,
+--by client-id uses just one.
+
+--ip reserves another address of the same subnet instead; the client
+moves there when it next renews.`,
 		Args: cobra.ExactArgs(1),
 		RunE: func(cmd *cobra.Command, args []string) error {
 			c, err := newClient()
@@ -153,6 +156,9 @@ its client ID (option 61): either one gets it the address. --by mac or
 			if by != "" {
 				body["by"] = by
 			}
+			if ip != "" {
+				body["ip"] = ip
+			}
 			var resp api.MutationResponse
 			if err := c.Do(ctx(), "POST", "/leases/"+args[0]+"/reserve", nil, body, &resp); err != nil {
 				return err
@@ -162,6 +168,7 @@ its client ID (option 61): either one gets it the address. --by mac or
 	}
 	reserve.Flags().StringVar(&hostname, "hostname", "", "the reservation's host name")
 	reserve.Flags().StringVar(&comment, "comment", "", "a comment")
+	reserve.Flags().StringVar(&ip, "ip", "", "reserve this address of the lease's subnet instead")
 	reserve.Flags().StringVar(&by, "by", "", "know the client by mac, client-id or both (the default)")
 	cmd.AddCommand(reserve)
 	return cmd