feat(gamestream): the ENet control port exists only while a pairing does (WP0)

rusty_enet — a c2rust-style transpile of C ENet, 158 unsafe sites — parsed
unauthenticated UDP on 47999 from GameStream startup, before any client had
ever paired: the host's entire pre-auth-reachable unsafe surface. Pairing
itself is HTTPS on nvhttp and never touches the port, so it now binds only
while the paired-client list is non-empty: a Gate in control.rs reconciles
the port to the list (armed only under --gamestream), pairing phase 4 brings
it up before the new client can /launch, and removing the last pairing tears
it down — a live client gets the same termination+disconnect farewell as a
host-side session end. A never-paired host on a hostile LAN exposes no ENet.

En route: the management API's unpair never called save_paired, so a restart
resurrected the client — and would now have silently re-opened the port; it
persists (the test now runs against a throwaway PUNKTFUNK_CONFIG_DIR so it
can't clobber a real paired.json). rusty_enet is pinned =0.4.0 per the WP,
left to the cargo-audit job to flag advisories against it.

Gate (amd64 container): clippy --all-targets -D warnings clean;
gamestream::control 6/6; mgmt::tests 37/37 incl. the regenerated
api/openapi.json. On-box .133 verification (ports/pair/stream) still owed.

api/openapi.json

@@ -53,7 +53,7 @@
           "clients"
         ],
         "summary": "Unpair a client",
-        "description": "Removes the client's certificate from the pairing store. Caveat: the nvhttp TLS layer\ndoes not yet reject unlisted certificates (`gamestream/tls.rs` accepts any well-formed\nclient cert — a planned hardening step), so until that lands this removes the client\nfrom the listing without severing its ability to reconnect.",
+        "description": "Removes the client's certificate from the pairing store (persisted — the removal survives a\nhost restart). Removing the last pairing also closes the GameStream ENet control port\n(UDP 47999), which is only bound while at least one pairing exists. Caveat: the nvhttp TLS\nlayer does not yet reject unlisted certificates (`gamestream/tls.rs` accepts any well-formed\nclient cert — a planned hardening step), so until that lands this removes the client\nfrom the listing without severing its ability to reconnect.",
         "operationId": "unpairClient",
         "parameters": [
           {