Numbers are identity; the reading order is a generated, checked index

Decided after measuring what renumbering actually costs: 96 references in code
comments across two repositories, none of which would have failed to compile.
They would have pointed at the wrong reasoning, which is worse than a broken
link because nothing reports it.

So a number identifies a record and never changes. It cannot also be a
position -- a position moves when the set changes, and an identity that moves
is not one.

The reading order moves into an index generated from each record's `topic:`.
Six topics, in the order somebody learns the system.

The index is WRITTEN rather than only generated on demand, which reverses what
this repository previously said. The reason it said otherwise is that a
hand-written index drifts -- but a reader looking at the folder on a forge sees
the folder, not a command, and the drift objection is answered by checking
rather than by refusing to write one. That is §5's own rule: a rule states how
it is checked.

Two checks, both confirmed to bite. index.py fails when the written order no
longer matches the records. records.py fails when a record has no topic or one
nobody defined -- the quiet failure being a record that vanishes from the order
rather than appearing in the wrong place.

02-DECISIONS/README.md

@@ -62,6 +62,62 @@ rather than guessing.
 
 ## Index
 
-The index is **generated, not maintained** — run the `hq-status` skill, which reads the
-frontmatter of every record. A hand-written index drifts from the folder it describes, and
-this one had already done so after a single addition.
+**A number identifies a record and never changes.** Records are referenced from outside this
+repository — code comments, commit messages — so a number that moves invalidates them silently.
+Renumbering once cost 96 references across two code repositories, and that is why the numbers
+are now fixed.
+
+So the folder is in creation order, and **the reading order lives here.** It is generated from
+each record's `topic:` and written, because a reader looking at the folder on a forge sees the
+folder rather than a command. The objection to a written index is that it drifts — which is
+answered by checking it rather than by refusing to write one:
+
+```
+python3 00-META/checks/index.py --write    regenerate
+python3 00-META/checks/index.py            fail if stale
+```
+
+<!-- index:start -->
+
+### What the mesh is
+
+- **0001** — [The mesh brokers capabilities; nodes host; agents think](0001-mesh-brokers-nodes-host-agents-think.md)
+- **0002** — [Nodes communicate over a message broker, not over HTTP](0002-nodes-communicate-over-a-broker.md)
+- **0003** — [An agent is a persistent employee, not an instance of a pool](0003-agents-are-persistent-employees.md)
+
+### Its tiers, from the bottom up
+
+- **0004** — [A node, and how it joins](0004-a-node-and-how-it-joins.md)
+- **0005** — [The node host](0005-the-node-host.md)
+- **0006** — [The substrate and the control plane](0006-the-substrate-and-the-control-plane.md)
+- **0007** — [Connectivity](0007-connectivity.md)
+- **0008** — [A context owns its store, exclusively](0008-a-context-owns-its-store.md)
+
+### What runs on them, and how it gets there
+
+- **0009** — [Modules and the graph](0009-modules-and-the-graph.md)
+- **0010** — [Delivery](0010-delivery.md)
+
+### How it is built
+
+- **0011** — [Managed files are generated onto nodes and never edited there](0011-managed-files-are-generated-never-edited.md)
+- **0012** — [The mesh creates no symlinks — a derived file is a copy](0012-the-mesh-creates-no-symlinks.md)
+- **0013** — [Schema and state changes are numbered migrations, in the same language as the code](0013-schema-changes-are-numbered-migrations.md)
+- **0014** — [No workspace — each module is a standalone package consuming published dependencies](0014-no-npm-workspace.md)
+- **0015** — [Applications live in their own repository; the monorepo is for the mesh](0015-applications-live-in-their-own-repository.md)
+- **0016** — [The lab](0016-the-lab.md)
+
+### How it is checked
+
+- **0017** — [A test defends a decision](0017-a-test-defends-a-decision.md)
+- **0018** — [A picture of a system is read from the system, never from what asked for it](0018-a-picture-is-read-from-what-runs.md)
+
+### How we work
+
+- **0019** — [How this repository works](0019-how-this-repository-works.md)
+- **0020** — [The mesh is governed by a constitution, injected where work is decided](0020-the-mesh-is-governed-by-a-constitution.md)
+- **0021** — [HQ is the source of the mesh constitution](0021-hq-is-the-source-of-the-constitution.md)
+- **0022** — [The constitution absorbs what is already enforced](0022-the-constitution-absorbs-what-is-enforced.md)
+- **0023** — [The approval is the checkpoint, not the second pair of hands](0023-approval-is-the-checkpoint.md)
+
+<!-- index:end -->