docs(book): a mod is a plugin too, and RCON is not the Rust answer

The kit taught, to an audience outside this org, that Rust needs "no game-side
plugin to write at all" because it ships RCON. That is overruled: the Rust dry
run reaches the game through a MOD - a plugin loaded by the server's own
framework, hooking events and dialling out - exactly as the ServUO overlay does
(docs#170).

Chapter 3's section kept its question and lost its example, which turned out to
improve it. The useful test is not "does my game expose a protocol" but "does it
DELIVER EVENTS": a remote-control channel is built for an operator typing
commands and tells you what you asked about, when you ask, and a website needs
what happened whether or not anyone was listening. A channel that answers
questions can only be polled, and polling turns "someone left the clan at 14:02"
into "the count was different at 14:03".

Rust now appears in that section as the counter-example rather than the example,
and carries the finding that is actually worth having: its server is a BINARY
where ServUO is source you compile, and the three-part shape survives that
unchanged. The plugin-dials-out arrangement is not a property of having source
access.

Chapter 4 said a game with a remote-control protocol may not need any of it, and
that its worked example is source you build. Both now say what is true - the
rules in that chapter are properties of being inside a game loop, and apply
identically to a mod in a closed server.

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

book/04-game-plugin.md

@@ -4,10 +4,17 @@ The chapter with the least code and the highest stakes. Everything else in this
 book fails by showing an operator a broken web page; this part fails by taking the
 game down while people are playing it.
 
-If your game already speaks a remote-control protocol, you may not need any of
-this — see the end of [chapter 3](03-sidecar.md). If it does not, something has to
-run inside the game and feed your sidecar, and the rules below are what keep that
-something from being the reason the server froze.
+If your game already **delivers events** on a surface of its own, you may not need
+any of this — see the end of [chapter 3](03-sidecar.md), and read the test there
+before deciding, because a channel that answers questions is not the same thing.
+Otherwise something has to run inside the game and feed your sidecar, and the
+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.
 
 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