docs(record): the implementation record for the documentation architecture

What the measurements say afterwards, what the review pass found, what the work
found while building the examples, and what is open with the reason each is open
rather than late.

The review section is the part worth keeping: fourteen cross-references in two
languages were left pointing at section numbers that had moved with their
sections, and every heading-structure and bilingual-parity check stayed green
throughout, because neither is a check about meaning. A mechanical move keeps
every structure valid and leaves the prose pointing at the old shape.

Author: speak-agent

.agents/docs/2026-09-08-documentation-architecture-three-trees.md

@@ -459,3 +459,112 @@ from the earlier plan and is where its example work rejoins.
    corrected in place by later measurement, which is how their own value was
    preserved. Making that a named, dated block at the end — rather than an edit
    in the body — would keep both properties.
+
+---
+
+## 12. Implementation record, 2026-09-08
+
+Shipped as PR #590 on `docs/architecture-three-trees`. Stages 1, 2, 3, 5 and 6
+of §8 are done; stage 4 (translating the specifications) and stage 7 (SPEC-005 /
+SPEC-006) are not, and §12.4 says why.
+
+### 12.1 What the measurements say afterwards
+
+| surface | before | after |
+|---|---|---|
+| `docs/05-mcpp-toml.md` | 3,129 lines | 1,626 |
+| largest chapter | 3,129 | 1,218 (`07`) |
+| `.agents/docs/README.md` | one heading | generated, 313 lines over 269 records |
+| user chapters citing a design record | 9 | 0 |
+| specifications listed on the front page | 2 of 4 | 4 of 4 |
+| stale `docs/NN-*.md` citations | 6 | 0 |
+| manifest keys with no example | 13 | 7 |
+| `build.mcpp` API names used by an example | 10 of 38 | 12 of 38 |
+| structure rules enforced in CI | 0 | 9 |
+
+The seven manifest keys still uncovered are the ones §3.2 classifies as a code
+block rather than an example — `[hooks]`, `scan_overrides`, `cxx_runtime`,
+`module_extensions`, `platforms`, `[feature-xlings]` — **plus two the plan
+intended to cover and did not**, for reasons worth stating rather than
+carrying forward as debt:
+
+- `[resources]` compiles on PE targets only, and a declared file must exist on
+  every target. An example for it would add a binary `.ico` to the tree and
+  then assert nothing on the two platforms most of this repository's CI runs
+  on. It belongs to a Windows-facing example, which does not exist yet.
+- `[runtime]` is the provider-neutral runtime contract a prebuilt library
+  declares. §4.2 folded it into the publishing example, which is blocked
+  (§12.4).
+
+### 12.2 What the review pass found
+
+Read as rendered text rather than as a diff, which is the rule the skill states
+for exactly this reason:
+
+**Fourteen cross-references, in both languages, left pointing at section numbers
+that had moved with their sections.** `§2.8.1` cited from inside the chapter
+that now contains it; `§2.13` cited from the chapter that now *is* it; `§2.14`
+cited from the chapter it moved to. Every heading structure stayed valid and the
+bilingual parity check stayed green throughout, because neither of those is a
+check about meaning.
+
+> A mechanical move keeps every structure valid and leaves the prose pointing at
+> the old shape. The checks cannot see it; a reader can.
+
+**Chapter 17's "Related chapters" ended up mid-chapter**, and its own bullet for
+`05` said "every manifest key, including `[xlings]`" — the one claim the move
+made false.
+
+**Two rules were written in the skill and not applied.** Chapter 22 had no
+"Current limitations" while the skill calls that section mandatory. And "every
+document states which rung it is on" was followed by no chapter and is not
+useful to a reader; it became "a `Related documents` line naming the rung above
+and below", which is what the chapters actually do and what carries the
+gradient. A rule nothing follows is worse than no rule, because it makes the
+skill unfalsifiable.
+
+### 12.3 What the work found in the implementation
+
+Three defects met while building `12-a-new-device-language`, each now in its
+README because a rule author will meet all three:
+
+- An action's command runs from the **build directory**, so a rule joins
+  `manifest_dir()` to the package-root-relative paths `device_sources()` gives.
+- A pipeline's exit status is its last command's. The first `toyc` summed with
+  `… | paste -sd+ - | bc`; when an earlier stage produced nothing `bc` still
+  exited 0, `set -e` never fired, and the program compiled, linked, ran and
+  printed `0`.
+- **The compiler is a declared input.** Without it, editing `toyc.sh` left every
+  edge clean and the artifact kept the previous compiler's bytes.
+
+And one in the tooling: `mcpp emit xpkg` produces a descriptor that
+`mcpp xpkg parse` rejects for a package keeping its own `mcpp.toml` — the
+emitted `mcpp` segment carries `manifest = "mcpp.toml"` and no `sources` list,
+and the validator requires one. Isolated by adding the list by hand, which makes
+it validate. Recorded in `docs/21` as a limitation; not fixed in a documentation
+change.
+
+`build_examples.sh` refused all four new example roots for being in neither
+`BUILD` nor `SKIP`. That is the denominator discipline working, and it is the
+same shape §7 gives the coverage check.
+
+### 12.4 What is open, and why each is open rather than late
+
+**The specifications' language** (§6.2, §11.1). English primary with a 简体中文
+mirror is the recommendation and is 1,156 lines of translation. It is an open
+question in this document, and translating before it is answered is the wrong
+order. What did ship is the half that is not a question: the register rules now
+cover `docs/specs/`, which they were exempt from by a glob rather than by a
+decision.
+
+**SPEC-005 and SPEC-006** (§6.1). Each is a promotion of text that exists and
+each is its own review.
+
+**A publishing round-trip example** (§4.3, B3). Blocked on the emit/parse
+mismatch above: the loop it would teach cannot be shown end to end while the
+descriptor `mcpp emit xpkg` writes is one `mcpp xpkg parse` refuses.
+`[resources]` and `[runtime]` were to be folded into that track and are
+therefore still uncovered.
+
+**14.7's `--no-accel` diagnostic and L0 of the rule ladder** are unaffected by
+this batch and remain where the heterogeneous plan records them.