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/roadmap.md

@@ -29,9 +29,10 @@ see [Status & Progress](/docs/status).
 
 ## ✅ Shipped
 
-- **The host, two ways.** A GameStream host any [Moonlight](/docs/moonlight) client can use, and the
-  lower-latency native [`punktfunk/1`](/docs/how-it-works) protocol (QUIC control + UDP data with
-  GF(2¹⁶) Leopard FEC + AES-GCM). Both run from one process.
+- **The host, two ways.** The lower-latency native [`punktfunk/1`](/docs/how-it-works) protocol (QUIC
+  control + UDP data with GF(2¹⁶) Leopard FEC + AES-GCM) — the secure default — and, opt-in via
+  `serve --gamestream`, a GameStream host any [Moonlight](/docs/moonlight) client can use. Both run
+  from one process.
 - **Native-resolution virtual displays** on Linux across KWin, GNOME/Mutter, gamescope, and
   Sway/wlroots, with a fully zero-copy GPU path to NVENC (stable 240 fps at 5120×1440).
 - **A native Windows host** (NVIDIA, x64) — a signed installer with secure-desktop capture and a