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.

00-META/checks/README.md

@@ -1,7 +1,9 @@
 # Checks
 
 ```
-python3 00-META/checks/records.py
+python3 00-META/checks/records.py      structure: links, citations, supersession, topics
+python3 00-META/checks/index.py        the reading order in 02-DECISIONS/README.md is current
+python3 00-META/checks/index.py --write  regenerate it
 ```
 
 Non-zero exit on any problem, so it can be a gate rather than a report.
@@ -22,6 +24,8 @@ indistinguishable from one that cannot.
 | `live-citation` | a governing document citing a **superseded** record names its replacement in the same paragraph | `01-to-be/README.md` citing ADR 0022 as live guidance |
 | `supersession` | if A says it was superseded by B, B says it supersedes A | ADR 0012 never declared that it superseded 0011 |
 | `numbering` | the number in the filename is the number in the heading | — |
+| `topics` | every record names a topic the index knows | — |
+| *(index.py)* | the written reading order matches what the records say | — |
 
 ## What is deliberately not checked