docs(book): name Oxide, and the question "how many sidecars" answers

Follows the org lead's two corrections on docs#170. The Rust example is an OXIDE
plugin - naming the framework is the difference between a design a reader can
start from and one they have to go and choose for themselves - and the
architecture pairs one sidecar to one game server, on that server's own host.

Chapter 3 gains the general form of that second one, since it is the chapter
where a reader decides what to build: if your game runs as a fleet, "how many
sidecars" is answered by where the loopback boundary is, not by how many
processes you would rather run. Your module holding several clients is the
cheaper end of that trade, and core never learns there is more than one.

Co-Authored-By: Claude <noreply@anthropic.com>

book/04-game-plugin.md

@@ -12,9 +12,9 @@ rules below are what keep that something from being the reason the server froze.
 
 **That something does not have to be source you compile.** The worked example
 below is an overlay built into a server whose code you have; the dry run for Rust
-is a **mod** loaded by a closed server's own framework, hooking published events.
-Every rule in this chapter applies identically to both — they are properties of
-being inside a game loop, not of how you got there.
+is an **Oxide plugin**, C# loaded by a closed server's own mod framework and
+hooking published events. Every rule in this chapter applies identically to both —
+they are properties of being inside a game loop, not of how you got there.
 
 The worked example is `servuo-plugins`, the Ultima Online shard plugin, whose link
 layer is one file: `overlay/Scripts/Custom/Bridge/BridgeLink.cs`. It is C# against