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/quickstart.md

@@ -22,9 +22,11 @@ Each one covers the NVIDIA driver, the dependencies, and how to build and run th
 From a terminal **inside your desktop session** (so the host can reach your compositor):
 
 ```sh
-punktfunk-host serve --native
+punktfunk-host serve
 ```
 
+This is the secure native-only default — the native `punktfunk/1` plane plus the web console. To also
+serve stock Moonlight clients, add `--gamestream` (trusted-LAN only; see [Moonlight](/docs/moonlight)).
 The host starts listening and prints its identity fingerprint. It advertises itself on your local
 network, so clients can find it by name. Leave it running. (To start it automatically at boot, see
 [Running as a Service](/docs/running-as-a-service).)