fix(cilium): point cgroup.hostRoot at the host's real cgroup2 root

hostRoot was /run/cilium/cgroupv2 with autoMount disabled, but nothing
mounts cgroup2 there. The agent mounted one from inside its own private
cgroup namespace, so socket-LB attached to the agent's own cgroup and
host processes got no Service/LB translation. k3s-prod-1 (Gateway L2
lease holder + kube-vip VIP) therefore could not reach the Gateway VIP,
and its apiserver's OIDC authenticator has failed since 2026-09-12.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

platform/cilium/values.yaml

@@ -41,11 +41,13 @@ hubble:
 # [parity] ingress VIP announcement on the flat LAN (docs/09, OQ-1)
 l2announcements:
   enabled: true
-# [parity] k3s provides the cgroupv2 mount; do not auto-mount
+# [parity] the host's cgroup2 root is /sys/fs/cgroup (systemd mounts it); do not auto-mount.
+# NOT /run/cilium/cgroupv2: nothing mounts cgroup2 there, so socket-LB attached to the agent's own
+# cgroup and host processes lost Service/LB translation (2026-10-03 — see helmrelease.yaml).
 cgroup:
   autoMount:
     enabled: false
-  hostRoot: /run/cilium/cgroupv2
+  hostRoot: /sys/fs/cgroup
 # [parity] roll the agent DaemonSet ONE node at a time. Overrides the cilium 1.20.1 chart
 # default of 2, which on 3 nodes lets a single bad render break 2/3 agents at once — as the
 # 2026-09-01 incident did (k8sServiceHost: "" → the `config` init container fell back to the