Record the board, which is the last designed document with no code

One reading answered three ways, holding nothing and touching no context's
store — which is the constraint the whole document is about, and the thing the
board being replaced gets wrong.

03-DESIGN/01-to-be/11-a-board.md

@@ -1,8 +1,10 @@
 ---
 layer: to-be
 status: designed
-code: []
-updated: 2026-08-30
+code:
+  - mesh-control cmd/mesh-control/board.go
+  - mesh-control cmd/mesh-control/readable.go
+updated: 2026-08-31
 decisions:
   - 02-DECISIONS/0008-a-context-owns-its-store.md
   - 02-DECISIONS/0001-mesh-brokers-nodes-host-agents-think.md
@@ -72,3 +74,36 @@ page that led with the third would bury the first:
 as it was and what is wrong is in what was sent; failed means it is in a state nobody declared and
 what is wrong is on the machine. They are fixed in different places, so a page that said "error"
 for both would send half its readers to the wrong one.
+
+## What was built
+
+*2026-08-31.*
+
+**One reading, three ways of saying it.** The questions are asked once, by one function, and
+answered as a person's `status`, as its JSON, and as this page. Three implementations of *which
+machine is not doing what it was told* would be three chances to disagree about it — and the
+disagreement would surface as two people looking at two screens arguing about which machine is
+broken.
+
+**It holds nothing and changes nothing.** Every request reads the mesh now. There is no cache to
+go stale, no table of what the mesh looked like last time, and no button: every action a board
+could offer already exists as a command, and one that did something no command does would be a
+second implementation of a decision.
+
+**It never touches a context's store.** That is the whole constraint above, kept: the board is a
+client of the same functions the commands use, so provisioning can change its tables without the
+change being weighed against a page.
+
+**A board that cannot read the mesh says so.** An empty page says *nothing is wrong* in the one
+situation where nobody can know that, so the failure is rendered instead — and it says explicitly
+that it is a statement about the page rather than about the mesh.
+
+**A machine's own words are shown, and are not markup.** They are the whole reason the page is
+useful — a board that said only *failed* would send a person to ask the thing they opened the
+board to avoid asking. They are also the only text on the page that nobody in this repository
+wrote, which is why the escaping is a test rather than an assumption.
+
+*Checked by giving a machine a declaration it cannot apply and requiring the page to name that
+machine, say `failed` rather than `error`, and quote what the host said — then by comparing the
+page's own JSON against the command's, because two answers to "which machine is broken" would be
+worse than either alone.*