fix(vpn-ingress): stop trying to set net.ipv4.ip_forward — a hostNetwork pod cannot, and wg-quick dies on a failed PostUp

The tunnel never came up on its first reconcile with a real key. One line of my
own manifest was the whole cause. Measured on prod 2026-10-06:

    [#] ip link add dev wg0 type wireguard        <- fine
    [#] wg addconf wg0 /dev/fd/63                 <- fine
    [#] ip -4 address add 10.90.0.1/24 dev wg0    <- fine
    [#] ip link set mtu 1420 up dev wg0           <- fine
    [#] sysctl -q -w net.ipv4.ip_forward=1
    sysctl: permission denied on key "net.ipv4.ip_forward"
    [#] ip link delete dev wg0
    **** Tunnel failed, will stop all others! ****

Not a missing capability: with hostNetwork the runtime mounts /proc/sys
READ-ONLY (confirmed in the pod's /proc/mounts), because those sysctls are the
HOST's globals. CAP_NET_ADMIN cannot lift a read-only mount, so the write can
never succeed here -- and wg-quick treats a failed PostUp as fatal, so it
deleted the interface it had just built.

The line was never needed. node_baseline already sets ip_forward=1, and reading
it from the pod returns 1. My header comment said exactly that ("so no pod
sysctl is needed") and then emitted the sysctl anyway, justified as "re-asserted
so this component does not silently depend on another role's sysctl file". That
defensive re-assertion is the entire outage. In a hostNetwork pod: VERIFY host
state, never write it.

Replaced with a read in the init container -- `grep -qx 1
/proc/sys/net/ipv4/ip_forward` -- failing closed with a message naming the
node_baseline role. Still fail-closed on purpose: a tunnel that comes up unable
to forward satisfies every probe here and silently drops all client traffic.

Also documents, in the manifest and the runbook, that the image's `chown:
Operation not permitted` run and its "we will not provide support for it"
warning are expected and harmless -- capabilities are dropped to NET_ADMIN so
its s6 init has no CAP_CHOWN, /config is a writable tmpfs, everything runs as
root, and wg-quick proceeds (it did; the failure was 30 lines later).

Three other unknowns closed by the same reconcile, now recorded in the README:
the kernel module reaches the pod (ip link add type wireguard works, no
CAP_SYS_MODULE needed); allowPrivilegeEscalation:false does NOT break the s6
init; and the image's iptables is nf_tables (xtables-nft-multi) matching Debian
13, which closes the worst documented fear -- rules landing in a table the
kernel never consults.

No sensitive paths; merges on a green gate with no override.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

platform/vpn-ingress/README.md

@@ -173,6 +173,18 @@ NetworkPolicy applies.
   (`latest handshake` under ~3 min means live). Never dump `wg showconf` — it prints the private key.
 * **Full bring-up and verification:** `runbooks/vpn-remote-access.md`.
 
+## Measured on prod, 2026-10-06 — what is now known rather than assumed
+
+The first real reconcile settled four things this component's design rested on. Three went the
+right way; one was a defect in my own manifest.
+
+| question | answer |
+|---|---|
+| Does the **kernel module** reach the pod? | **Yes.** `ip link add dev wg0 type wireguard` succeeds, and `/proc/modules` lists `wireguard`, after the `node_baseline` run. `CAP_SYS_MODULE` is correctly not needed. |
+| Does `allowPrivilegeEscalation: false` break the image's **s6 init**? | **No.** The container reaches `wg-quick` normally. The flag stays. |
+| Is the image's **iptables** backend compatible with the host's? | **Yes — both nft.** The image ships `iptables v1.8.13 (nf_tables)` via `xtables-nft-multi`, and Debian 13 is nf_tables. This was the documented worst fear (rules silently landing in a table the kernel does not consult); it is closed. |
+| Can the pod set **`net.ipv4.ip_forward`**? | **No, and it must not try.** `/proc/sys` is mounted **read-only** under `hostNetwork` (`proc /proc/sys proc ro,…`), because those are the host's globals; `sysctl -w` fails with EPERM whatever the capabilities. A failed `PostUp` is fatal to `wg-quick`, so this one line took the whole tunnel down. The node already has it set to `1`; the init container now **verifies** rather than writes. |
+
 ## Prerequisites outside this directory
 
 1. **The `wireguard` kernel module on every node.** Added to `node_baseline_kernel_modules`