Keep the AS list when the asupdate generator fails (closes #173)

make asupdate piped the generator into gzip, so the target saw gzip's
exit status. A failed download replaced pkg/asinfo/asdata.json.gz with
an empty file and the target still exited 0.

The target now runs the new script/asupdate. It writes the output of
cmd/asinfo-gen to a temporary file. Only when the generator succeeds
does it compress the list and move it into place. Otherwise it leaves
the file alone and exits non-zero. The generator now fails when as.csv
holds no AS.

The generator reads as.csv from ipverse/as-metadata, the source's new
name. The list it builds there is unchanged, so no new list is
committed.

Model: opus-5-5

README.md

@@ -110,9 +110,10 @@ Every route answers `HEAD` as it answers `GET`, without the body.
       `handle` and `description`
 
     The search reads the AS list built into RouteWatch, not the database. The
-    list comes from [ipverse/asn-info](https://github.com/ipverse/asn-info). A
-    match can therefore be an AS that the database has no record of. A `q` that
-    is missing, empty or only spaces answers 400.
+    list comes from
+    [ipverse/as-metadata](https://github.com/ipverse/as-metadata). A match can
+    therefore be an AS that the database has no record of. A `q` that is
+    missing, empty or only spaces answers 400.
 
 A JSON answer is `{"status": "ok", "data": ...}`, or for an error
 `{"status": "error", "error": {"msg": ..., "code": ...}}`, and carries an
@@ -485,6 +486,11 @@ provide:
   or `go.sum`) and `go fmt` (writes), then `script/check`
 - `script/install-precommit` — install the git pre-commit hook that runs
   `script/precommit`
+- `script/asupdate` — rebuild the AS list built into RouteWatch,
+  `pkg/asinfo/asdata.json.gz`, from the `as.csv` file of
+  [ipverse/as-metadata](https://github.com/ipverse/as-metadata). It needs the
+  network. It runs `cmd/asinfo-gen`, which fails when it finds no AS. When
+  `cmd/asinfo-gen` fails, the old list stays and the script exits non-zero.
 
 Tests and the linter run only in Docker, as the `test` and `lint` phases of the
 `Dockerfile`; the image cannot be built unless both pass. Every `docker build`