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>

scripts/windows/host.env.example

@@ -23,9 +23,10 @@ PUNKTFUNK_SECURE_DDA=1
 # Log level (info | debug | trace). Logs land in %ProgramData%\punktfunk\logs\.
 RUST_LOG=info
 
-# The host subcommand the service launches. Default: `serve --native` (GameStream/Moonlight + the
-# native punktfunk/1 QUIC host in one process). Uncomment to override.
-#PUNKTFUNK_HOST_CMD=serve --native
+# The host subcommand the service launches. Default: `serve --gamestream` (native punktfunk/1 host
+# ALWAYS on + the GameStream/Moonlight-compat planes). Use `serve` for a SECURE native-only host
+# (no plain-HTTP pairing / legacy GCM nonce reuse — security-review #5/#9). Uncomment to override.
+#PUNKTFUNK_HOST_CMD=serve --gamestream
 
 # Multi-GPU boxes only: force the NVENC/Desktop-Duplication GPU by Description substring. Leave
 # unset on single-GPU machines (the default auto-picks the discrete adapter).