fix(sidecar): refuse a plugin that names another server (D155)

`[game].server_id` was a cross-check that WARNED and kept the plugin's id.
The phase 18 walk showed what that costs: a second server's plugin,
parked in this listener's backlog by a plugin bug (Rust-Plugins, D154),
was accepted the moment the first server's plugin reloaded, and the
website showed server "alpha" with beta's hostname and wipe.

Now, with `server_id` set, the connection is closed on the first frame
that names another server, BEFORE that frame reaches the store or the
feed, and both ids are logged at ERROR. The command channel is installed
only once a frame has named this server, so no website command (a grant,
a world write) can reach a plugin about to be refused, and /health reports
the plugin connected only from then. Blank `server_id`: nothing checked,
as before.

The egg's launcher now hands the sidecar the plugin config's ServerId once
that file exists. The plugin reads RUSTLINK_SERVER_ID only at its first
config write (D150); without this, a variable edited after the first boot
would be refused instead of changing nothing, as INSTALL.md promises.

Walked: a plugin aimed at another server's sidecar is refused with an
ERROR naming both ids, and the site keeps the right server's identity.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E14m6SuuY6i1vASFeGDBeY

sidecar/src/config.rs

@@ -17,9 +17,11 @@
 //! R8 makes the platform multi-server: one sidecar per game server, and every row the module
 //! stores carries the server it came from. The plugin declares its own `serverId` in `server.hello`
 //! and that is the authority. This setting is a **cross-check**, not a second source of truth: when
-//! both are set and they disagree, the sidecar logs the disagreement loudly and keeps the plugin's.
-//! Two servers pointed at one sidecar by a copy-pasted config is the mistake this catches, and it
-//! is silent in every other design.
+//! both are set and they disagree, the sidecar **refuses the plugin** — it closes the connection
+//! before the frame is filed, and logs both ids at ERROR (D155). Two servers dialling one sidecar is
+//! the mistake this catches, and it is silent in every other design. It was a warning that kept the
+//! plugin's id until the phase 18 walk showed what that costs: one server's history filed under
+//! another's name, visible on the website. Left blank, nothing is checked.
 
 use std::env;
 use std::fs;