feat(host): GameStream/Moonlight compat is now opt-in (--gamestream) — secure native-only by default

Follows the security audit (#5/#9): the GameStream-compat plane carries inherent on-path weaknesses
that can't be fixed on the wire without breaking stock Moonlight — its pairing runs over plain HTTP
(#9, MITM-able during the pairing window) and its legacy control encryption can reuse GCM nonces (#5,
a passive eavesdropper can recover/forge input). The native punktfunk/1 plane (SPAKE2 PIN pairing +
per-direction AEAD nonces) has neither. So flip the default to secure-by-default:

- `serve`              → native punktfunk/1 plane + management API ONLY (no GameStream surface).
- `serve --gamestream` → ALSO the GameStream/Moonlight-compat planes (nvhttp pairing, RTSP, ENet
  control, _nvstream mDNS). Opt-in, logged with a trusted-LAN caveat. `--moonlight` is an alias.
- The native plane is now ALWAYS on in `serve` (`--native` is a kept-for-compat no-op); the unified
  GameStream+native host is `serve --gamestream`.

`gamestream::serve` gates the GameStream spawns (nvhttp/rtsp/control/mdns) on the flag; the native
plane + mgmt + native-pairing handle always run.

To avoid silently regressing validated Moonlight deployments, the explicit deployment configs PRESERVE
Moonlight via `--gamestream` (each documents dropping it for a secure native-only host): the Linux
systemd unit, the Steam Deck installer, and the Windows service default (DEFAULT_HOST_CMD). The bare
`serve` default (new/manual use) is secure.

Docs swept to match (host-cli, moonlight, quickstart, install, packaging READMEs, CLAUDE.md, README,
…): Moonlight setup now instructs `--gamestream`; native/console refs use bare `serve`. OpenAPI
regenerated (a stale "run `serve --native`" string). fmt + clippy clean; 94 host tests green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs-site/content/docs/moonlight.md

@@ -11,10 +11,20 @@ a browser, a smart TV, or any device without a native client.
 > discovery/pairing — including **Windows** and **Android** (phone and Android TV). See
 > [Clients](/docs/clients) before reaching for Moonlight.
 
-## 1. Make sure the host is running
+## 1. Make sure the host is running with GameStream enabled
 
-On the host machine, `serve --native` (or your [service](/docs/running-as-a-service)) should be up.
-The host advertises itself on the network, so Moonlight usually finds it on its own.
+Moonlight needs the GameStream planes, which are **opt-in**. Run the host with `--gamestream`:
+
+```sh
+punktfunk-host serve --gamestream
+```
+
+(Bare `serve` is the secure native-only default and stock Moonlight clients can't connect to it; the
+native plane is always on, and `--gamestream` adds the Moonlight-compat surface.) GameStream pairs over
+plain HTTP and its legacy control encryption is weaker than the native plane's, so only enable it on a
+**trusted LAN**. If you run the host as a [service](/docs/running-as-a-service), make sure its
+`ExecStart` includes `--gamestream`. The host advertises itself on the network, so Moonlight usually
+finds it on its own.
 
 ## 2. Add the host in Moonlight