docs: the README states three tiers, both languages state the same targets, and a check enforces it

The two top-level READMEs were outside every documentation check but the emoji
rule, and had drifted from the implementation in three ways.

The target table now carries the three tiers the engine actually stores --
`verified`, `preview`, `planned` -- and every canonical spelling in
`modules/toolchain-model/src/triple.cppm`. The distinction is load-bearing:
a build for a `planned` row is refused, while a `preview` row builds and links,
and three rows the README called `planned` were `preview`. Three rows were
absent altogether. The 简体中文 table was seven rows behind -- every bare-metal
row, plus the footnote that says a freestanding target takes its C library,
startup code and emulator from a board-support package -- so that README
described a tool with no freestanding support at all.

The Feature Overview gains the surfaces shipped since 2026.8 and previously
unmentioned in either language: cross-compilation, bare metal and devices;
heterogeneous builds and accelerators; and the extension model of `build.mcpp`
and rule packages. `--release`, `--profile`, `--features`, `mcpp emit sbom` and
the machine-readable output join the command list, and `mcpp why` regains its
fourth topic.

The Documentation section names the parts of the manual and points at the index
that owns the reverse lookup, rather than naming seven chapters out of
thirty-one. The benchmark keeps its table, its provenance and its control row,
and loses half its lines. Two counts that rot with the tree -- "43+ C++23
modules" against 119 interface units -- are replaced by prose; the one count
that is a measurement stays where it is measured.

Rules 9 and 10 of `check_docs_structure.sh` now cover the README pair: the row
counts are 61 and 61, and a row dropped from either side fails the check. Rule
14 is new -- a link labelled with a chapter number must point at that chapter.
It was written for `[docs/13 -- ...](docs/40-baremetal.md)` in README.md and
found six more of the same, left in `docs/` and `docs/specs/` by the
renumbering in #590; all seven are corrected here.

`triple.cppm`'s own tier legend named two of the three tiers it stores. The
README now cites that file, so it is corrected in the same change.

Author: speak-agent

.github/tools/check_docs_structure.sh

@@ -17,6 +17,7 @@
 #  11. every chapter states its reader, its question and its exclusions
 #  12. a citation naming a section lands in the chapter that contains it
 #  13. every table the manifest reference documents is in the lookup index
+#  14. a link labelled with a chapter number points at that chapter
 #
 # What it deliberately does NOT check: whether a chapter documents what is
 # implemented, whether an assertion's strength matches its evidence, or whether
@@ -144,13 +145,17 @@ for f in .agents/docs/[0-9]*.md; do
     || bad "$f: front matter declares no valid \`status\` (active | landed | superseded | abandoned)"
 done
 
-# ── 9. every relative link in docs/ and examples/ resolves, fragment included ─
+# ── 9. every relative link in docs/, examples/ and the READMEs resolves ──────
 #
 # Rule 3 catches `docs/NN-*.md` named anywhere, including from source comments.
 # This is the other half: a Markdown link in a document that points at a file
 # which is not there. Both halves are needed -- a chapter moved in this batch
 # would satisfy one and break the other.
 #
+# THE TWO TOP-LEVEL READMEs ARE IN THE SET. They are the entry point to every
+# tree below them and they carry more relative links than most chapters, and
+# until they were added here nothing checked one of those links at all.
+#
 # THE FRAGMENT IS PART OF THE LINK. The first version of this rule discarded
 # it (`(?:#[^)]*)?`), so a link to a section that had been renamed resolved to
 # the file and was reported correct. Renaming 100 headings for register in one
@@ -195,7 +200,9 @@ def anchors_of(path):
         out.add(s if n == 0 else f"{s}-{n}")
     return out
 
-files = list(pathlib.Path("docs").rglob("*.md")) + list(pathlib.Path("examples").rglob("*.md"))
+files = (list(pathlib.Path("docs").rglob("*.md"))
+         + list(pathlib.Path("examples").rglob("*.md"))
+         + [pathlib.Path("README.md"), pathlib.Path("README.zh-CN.md")])
 cache = {}
 bad = 0
 for f in files:
@@ -227,11 +234,19 @@ PYCHECK
 # 简体中文 `[features]` section had no body at all, and 简体中文 §2.11 was
 # missing the `identity` verdict table. Both predate this check and both are
 # invisible to every other one.
+#
+# THE PAIR AT THE ROOT IS CHECKED TOO, AND IT IS WHERE THE COST WAS HIGHEST.
+# `README.zh-CN.md` carried 14 target rows against the English 21: the seven it
+# lacked were every bare-metal row, so a reader of the 简体中文 README saw a
+# tool with no freestanding support at all. Heading count, code-block count and
+# `<details>` count were all equal, which is why every other check was green.
 python3 - <<'PYPARITY' || fail=1
 import pathlib, sys, re
 bad = 0
-for en in sorted(pathlib.Path("docs").glob("*.md")):
-    zh = pathlib.Path("docs/zh") / en.name
+pairs = [(en, pathlib.Path("docs/zh") / en.name)
+         for en in sorted(pathlib.Path("docs").glob("*.md"))]
+pairs.append((pathlib.Path("README.md"), pathlib.Path("README.zh-CN.md")))
+for en, zh in pairs:
     if not zh.exists():
         continue
     def count(f):
@@ -330,6 +345,36 @@ for k in missing:
 sys.exit(1 if missing else 0)
 PYLOOKUP
 
+# ── 14. a link labelled with a chapter number points at that chapter ─────────
+#
+# Rule 3 checks that a named path exists and rule 9 that a link resolves. Both
+# passed on `[docs/13 -- Bare-Metal and Freestanding Targets](docs/40-baremetal.md)`
+# in README.md: the renumbering rewrote the path and left the label, so the
+# README told its reader to read chapter 13 for eleven of the twenty-one rows
+# in its own target table. A label that names a number is an assertion about
+# where the reader is being sent, and it is checkable against the path.
+python3 - <<'PYLABEL' || fail=1
+import re, pathlib, sys
+LINK = re.compile(r"\[([^\]]+)\]\((?!https?:|mailto:)([^)\s#]+)(?:#[^)\s]+)?\)")
+NUM  = re.compile(r"(?:docs/|^|[^0-9a-zA-Z])(\d{2})(?:\s*(?:--|—|-|\s)|$)")
+files = (list(pathlib.Path("docs").rglob("*.md"))
+         + [pathlib.Path("README.md"), pathlib.Path("README.zh-CN.md")])
+bad = 0
+for f in files:
+    for m in LINK.finditer(f.read_text(errors="ignore")):
+        label, path = m.group(1), m.group(2)
+        base = pathlib.Path(path).name
+        target_no = re.match(r"(\d{2})-", base)
+        label_no = NUM.match(label.strip())
+        if not target_no or not label_no:
+            continue
+        if target_no.group(1) != label_no.group(1):
+            print(f"FAIL: {f}: label `{label}` names chapter "
+                  f"{label_no.group(1)}, the link goes to {base}")
+            bad += 1
+sys.exit(1 if bad else 0)
+PYLABEL
+
 if [[ "$fail" -eq 0 ]]; then
   echo "OK: docs structure checks pass"
 fi