docs(design): record the two decisions of this batch

Section 14: the register rules were written and then not enforced. The check
had two independent defects, each a form recorded elsewhere -- the predicate
tested a marker (`?`) rather than the property (an interrogative word), and the
object set was enumerated syntactically (lines beginning with `#`) while a
column header is a heading by every property that matters. Extending both
reported 100 headings and 57 column headers. It also surfaced that the
structure check discarded a link's fragment, so a link to a renamed section
resolved to the file and passed. R5 is recorded with it: the scope of a
background section is decided by the problem, not by the tool.

Section 15: the extension model had no owner. Chapters 30 and 31 held every
primitive between them and no section stated the model itself. Chapter 31 now
opens with it, and its boundary is measured rather than asserted. The example
had to become real before it could carry it -- the language, the compiler, and
the tool-store boundary it measured.

Author: speak-agent

.agents/docs/2026-09-08-the-documentation-as-a-book.md

@@ -538,3 +538,97 @@ question and exclusions; on the front door that block reads as machinery. `00`
 has no "not here" to declare because everything else *is* elsewhere, which its
 closing paragraph says in a sentence instead. Rule 11 exempts `00` by name, with
 that reason in the script.
+
+## 14. The register rules were written and then not enforced
+
+The user read chapter 00 and named three things: `背景:模块到了,工具链没跟上`,
+`谁在为这个落差付账`, and the table header `大致相当于谁的活`. All three are
+colloquial, all three violate §四 of the skill, and all three passed
+`check_docs_style.sh`.
+
+Two independent defects in one check, and each is a form recorded elsewhere:
+
+**The predicate tested a marker rather than the property.** It matched `?`, 吗
+and 呢 — the punctuation a question usually carries. The property is the
+interrogative *word*, and neither `谁在为这个落差付账` nor `打什么由谁决定`
+carries the marker. The predicate is now the word set 谁 / 哪 / 什么 / 多少 /
+为何 / 如何 / 怎样 / 怎么.
+
+**The object set was enumerated syntactically.** It read lines beginning with
+`#`. The property a heading has — it names a topic, it is read out of order, it
+is what a reader scans — is held equally by a table's header cells, which were
+therefore never examined. Rule 4 adds them: the row above a `|---|---|`
+separator, cell by cell.
+
+Extending both reported 100 headings and 57 header cells across 20 chapters,
+in both languages. They are now noun phrases. The rename table is in the commit
+rather than here, because it is a list of 157 strings and not a decision.
+
+**One consequence was not obvious.** Renaming 100 headings invalidates any link
+that names a section by its anchor, and rule 9 of `check_docs_structure.sh`
+discarded the fragment (`(?:#[^)]*)?`) — a link to a renamed section resolved to
+the file and passed. It now computes GitHub's slug, including the `-1` / `-2`
+suffix for repeated headings, and verifies the fragment. It immediately reported
+two anchors in chapter 30 that had been wrong since they were written, before
+any rename.
+
+### The rule that generalises
+
+R5 in the skill: **the scope of a background section is decided by the problem,
+not by the tool.** Chapter 00's background was about modules, which is the part
+of the problem mcpp is closest to. The problem a reader actually has is that a
+C++ project needs four things at once — a build description, dependencies, a
+compiler new enough, and an environment the result runs in — and no single tool
+owns all four. CMake is the de facto standard for the first, and a de facto
+standard is a statement about adoption rather than about the experience of use.
+The environment is the layer that fails most often because it is the only one
+nothing checks. Modules are the fourth constraint on that structure, not the
+structure.
+
+## 15. The extension model had no owner, and the example that shows it was a toy
+
+The user asked where the plugin system is documented, and whether it reaches
+preprocessing and languages of one's own. The tree had chapter 30 (build
+programs) and chapter 31 (rule packages), and between them every primitive —
+but no section stating the model: what the extension points are, what the
+ecosystem has built from them, and where the boundary is. R1 says a topic has
+exactly one owner, and this one had none.
+
+Chapter 31 now opens with it: five extension points with their effect and where
+each is declared; seven things the ecosystem has built from them; three shapes
+the model expresses (a new language whatever compiles it, preprocessing and
+code generation, and a file that is partly C++ and partly another language);
+and the boundary, which is measured rather than asserted —
+
+- a dependency's `device_extensions` is consulted *after* the built-in roles,
+  so a rule package cannot claim `.cpp`. Adding `".cpp"` to a rule's list is
+  not diagnosed and has no effect, measured;
+- module-interface extensions are `[build] module_extensions`, a project axis,
+  because a module interface is scanned, produces a BMI and joins the link —
+  three engine behaviours rather than a command;
+- an extension in neither table is refused by name, quoted from the run.
+
+### The example had to become real before it could carry this
+
+`examples/12-a-new-device-language` had a `.toy` that was "one integer per line,
+and the entry point returns their sum", compiled by a shell script. It
+demonstrated the two manifest keys and nothing else, and it was not a shape any
+ecosystem author meets.
+
+`.toy` now has `let`, assignment, `if`/`else`, `while`, calls between kernels,
+and the arithmetic and comparison operators. Its compiler is an ordinary mcpp
+package — lexer, recursive-descent parser, semantic checks, C++ emitter — built
+**for the build machine** through `tools = ["toyc"]` + `reexport = true` and
+reached with `mcpp::dep_bin`. That makes it the tree's first example of a
+dependency producing a host tool, a capability that until now existed only in
+prose.
+
+**And it measured a boundary the documentation did not state.** The tool store's
+key is the tool package's identity, version, host triple, compiler identity,
+profile, features and the versions of its transitive dependencies. It holds no
+source content, so editing a `path` tool's sources without changing its version
+leaves the cached binary in place: `mcpp run` reported `Finished dev in 0.00s`
+and printed the previous answer. Chapter 30 now states it, with the two ways
+out. The action itself is not the gap — the rule declares the compiler binary
+as an input, so a changed binary dirties the edge. What does not happen is the
+rebuild that would change those bytes.