feat: freeze ssh and vnc port contracts

Define the minimum machine networking surface for the next host slice and
limit exposed guest ports to SSH and VNC.

contract/networking.go

@@ -0,0 +1,20 @@
+package host
+
+type MachinePortName string
+
+type PortProtocol string
+
+const (
+	MachinePortNameSSH MachinePortName = "ssh"
+	MachinePortNameVNC MachinePortName = "vnc"
+)
+
+const (
+	PortProtocolTCP PortProtocol = "tcp"
+)
+
+type MachinePort struct {
+	Name     MachinePortName `json:"name"`
+	Port     uint16          `json:"port"`
+	Protocol PortProtocol    `json:"protocol"`
+}