fix(vpn): default client profiles to MTU 1280; correct the VPN runbook

new-peer.sh generated MTU 1420, which stalled traffic on mobile data
despite a healthy handshake (one phone, one carrier, 2026-11-09). Default
to 1280, overridable through WG_MTU.

Correct stale claims in the runbook and the DaemonSet header against
live BPF conntrack: LoadBalancer VIPs see 10.90.0.x as world, not a
SNATed host identity. Add a troubleshooting section.

The DaemonSet render is unchanged, so WireGuard pods do not roll.

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

platform/vpn-ingress/README.md

@@ -112,7 +112,9 @@ no change. Three further problems vanish with the host netns:
   `conformance/tests/l0-static/l2-announced-vips-serve-from-any-node`) and no
   `externalTrafficPolicy` trade-off to make at all.
 * **No double encapsulation.** Client UDP lands on the node's own NIC, so the underlay is a flat
-  1500-byte path, not Cilium's 1450-byte VXLAN. That is what makes `MTU = 1420` simply correct.
+  1500-byte path, not Cilium's 1450-byte VXLAN. That is what makes `MTU = 1420` correct on the
+  server's `wg0`. It says nothing about a client's own path: `new-peer.sh` defaults client profiles
+  to `1280` (see "Troubleshooting" in `runbooks/vpn-remote-access.md`).
 
 `hostNetwork` is *permitted* here, and not by oversight:
 `platform/kyverno/policies/restrict-host-namespaces.yaml` matches `namespaceSelector:

platform/vpn-ingress/daemonset.yaml

@@ -30,9 +30,12 @@
 # `cluster` entity, so VPN clients would not have been able to resolve `*.${BASE_DOMAIN}` either.
 # The VPN would have looked up and been useless, which is the worst failure shape available.
 #
-# In the host netns there is no pod identity to carry: forwarded client traffic is SNATed to the
-# NODE's address, so it reaches the Gateway, PiHole and every LAN host as `host`/`remote-node` —
-# exactly what a LAN client looks like. Three further problems disappear with it:
+# In the host netns there is no pod identity to carry, so a client never looks like a pod. What a
+# destination sees depends on how the packet is handled. LAN hosts and the internet: forwarded,
+# then MASQUERADEd to the NODE's address by the PostUp rule below. LoadBalancer VIPs: Cilium's load
+# balancer takes the packet as it enters from wg0, before that rule, so the source is still
+# 10.90.0.x with identity `world`, like a LAN browser (measured 2026-11-09 for PiHole .23 and Plex
+# .24; see README). Three further problems disappear with it:
 #   * net.ipv4.ip_forward is ALREADY 1 on these nodes (node_baseline writes it to
 #     /etc/sysctl.d/99-k3s-cilium.conf), so no pod sysctl is needed — and `net.ipv4.ip_forward`
 #     is NOT in the kubelet's safe-sysctl allowlist, so a pod-netns version of this would have