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/steamdeck/install.sh

@@ -170,7 +170,9 @@ fi
 # --- 5. systemd user services ---------------------------------------------
 log "Installing systemd user services"
 mkdir -p "$UNITS"
-SERVE_ARGS="serve --native --mgmt-bind 0.0.0.0:$MGMT_PORT"
+# --gamestream keeps the Moonlight-compat planes (the Deck commonly streams to Moonlight too); drop
+# it for a secure native-only host (no #5/#9 surface — native clients only).
+SERVE_ARGS="serve --gamestream --mgmt-bind 0.0.0.0:$MGMT_PORT"
 [ "$OPEN" = 1 ] && SERVE_ARGS="$SERVE_ARGS --open"
 cat > "$UNITS/punktfunk-host.service" <<EOF
 # Generated by scripts/steamdeck/install.sh — punktfunk Steam Deck host (native binary).