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

README.md

@@ -12,7 +12,7 @@ Namespace: `dhcp-system`. API group: `kea.unkin.net/v1alpha1`.
 |------|---------|
 | **KeaCluster** | Spawns a StatefulSet of `kea-dhcp4` + `kea-ctrl-agent` pods, an HA control channel, and an anycast DHCP `Service`. Holds global config (domain, lease times, NTP, PXE option-defs). |
 | **KeaSubnet** | One DHCPv4 subnet referenced to a KeaCluster (`clusterRef`). CIDR, optional pools, routers, DNS, domain, next-server. Pool-less subnets are declared so relayed requests are still serviced. |
-| **KeaClientClass** | PXE boot class matched on client architecture (option 93), e.g. `Legacy` → `/undionly.kpxe`, `UEFI-64` → `/ipxe.efi`. |
+| **KeaClientClass** | PXE boot class matched on client architecture (option 93), e.g. `legacy` → `/undionly.kpxe`, `uefi-64` → `/ipxe.efi`. |
 | **KeaAPI** | Spawns the Terraform-friendly REST API service (see below). |
 
 The **KeaCluster** controller lists the matching subnets and client classes,