docs(book): the four chapters — Phase 5 slice 2

The book, written out of the tree slice 1 proved. Four chapters in the order the
work happens: the first module in twenty minutes, the website module, the sidecar,
and the game-side plugin.

Shape, settled with the org lead:

  * template/README.md stays the REFERENCE — it travels with a copied template and
    CI holds it against the tree — and chapter 1 is the narration: what you should
    see after each step, the state your module lands in, and the four ways it fails.
    The chapter links to the checklist rather than restating it.
  * chapters 3 and 4 cite link/ and servuo-plugins/ by FILE AND IDENTIFIER, never by
    line. Those repositories move for their own reasons and checkLinks already
    forbids commit permalinks, so a line number in this book is wrong the moment
    they do. The template stays the only code quoted verbatim.
  * one PR: the outline's status table and the link check are only coherent when the
    whole set lands.

scripts/checkChapterPaths.js is the anti-rot half a machine can answer: every path
a chapter names in backticks must exist. None of those mentions is a markdown link,
so checkLinks never looked at them, and none is code, so nothing else did either —
renaming one template file would have left four chapters quietly pointing at
nothing. Its anchor list is STATED rather than derived from the tree, for the reason
the template's own build guard states it: a list derived from what exists cannot
fail when what exists changes, and an anchor that stops matching is a check that has
silently stopped checking. So each anchor must exist or the check fails. Eleven
tests, every "must not catch" case a span that really appears in the book.

stripFences moved to scripts/lib/markdown.js and both checks use it — shared code,
not a shared description.

CHAPTER 1 WAS RUN, NOT REASONED ABOUT. The template was copied into a real core on
edge, booted against the dev database, and every claim in "what you should see"
checked: the five log lines, /examplegame/status with its injected
<script type="module" src="/modules/examplegame/entry.js">, the chunk served
no-cache while module.json 404s, /api/v1/public/world/status, the capabilities in
/api/v1/public/modules, and the route in the merged /api/docs.json. Then the three
failures the chapter tells a reader to cause on purpose, because a chapter that
predicts the wrong debugging heuristic is worse than one that predicts none:

  * an undeclared prefix  -> stage `register`, "declared public/extra but never
    registered it", routes 404 and absent from /public/modules;
  * a table without the id prefix -> stage `schema`, at LOAD time, before mounting;
  * a throwing onBoot -> after mounting, so the same route answers 503 "Module
    unavailable" rather than vanishing.

All three came out exactly as written, and the messages in the chapter are that
core's own. Two small corrections fell out of the run: the log sample now shows the
real interleaving of core's three lines with the module's two, and the section on
failure adds that a module disappears from /api/v1/public/modules in every failure
case — a check that needs no login.

MODULE_SYSTEM.md 2.11.1 slice 2. Docs half: docs#146.

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

README.md

@@ -84,9 +84,14 @@ scripts/     the checks CI runs over both
 
 CI clones core at a **pinned commit**, asserts the version the template declares
 still matches that core's `MODULE_API_VERSION`, builds the template and runs its
-guards, checks every link in the book, and holds the template's rename checklist
-against the template's own tree. So a change to the contract breaks this repo's
-build loudly instead of leaving a chapter quietly wrong.
+guards, checks every link in the book, holds the template's rename checklist
+against the template's own tree, and checks that every path a chapter names is
+still there. So a change to the contract breaks this repo's build loudly instead
+of leaving a chapter quietly wrong.
+
+None of that can tell you whether a paragraph has become untrue about a file that
+still exists. That is a reviewer's job on every pull request, and a
+`MODULE_API_VERSION` bump is when it is owed in full.
 
 ## Licence