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/0018-a-picture-is-read-from-what-runs.md

@@ -1,4 +1,5 @@
 ---
+topic: checking it
 status: accepted
 date: 2026-08-24
 deciders: jochen