Fix DHCP sample gateways and lowercase PXE client-class names

Two sample bugs surfaced while authoring the argocd deployment.

- set subnet gateways to match the authoritative puppet hieradata: 198.18.13-16
  routers are .254 (not .1); 198.18.17 stays .1
- rename KeaClientClass samples Legacy/UEFI-64 to legacy/uefi-64 so they are
  valid RFC1123 object names (the operator renders the kea class name from
  metadata.name, which k8s forces to lowercase); align the README

Claude-Session: https://claude.ai/code/session_01JUoARVdmhxKQHyyyp1pxeT

config/samples/02-clientclasses.yaml

@@ -1,9 +1,10 @@
 # PXE boot classes matching the client architecture option (code 93), replacing
-# the legacy dhcpd "Legacy" and "UEFI-64" classes.
+# the legacy dhcpd "Legacy" and "UEFI-64" classes. Object names must be RFC1123
+# (lowercase); the operator renders the kea client-class name from metadata.name.
 apiVersion: kea.unkin.net/v1alpha1
 kind: KeaClientClass
 metadata:
-  name: Legacy
+  name: legacy
   namespace: dhcp-system
 spec:
   clusterRef: pxe
@@ -13,7 +14,7 @@ spec:
 apiVersion: kea.unkin.net/v1alpha1
 kind: KeaClientClass
 metadata:
-  name: UEFI-64
+  name: uefi-64
   namespace: dhcp-system
 spec:
   clusterRef: pxe