Commit the Alpine.js tarball in 3p/ and extract it at build time (closes #345)

The build no longer downloads Alpine.js. Its npm package tarball is committed as 3p/alpinejs-3.14.9.tgz, byte for byte the file script/fetch-assets downloaded, with the sha256 that script pinned. script/assets (make assets) extracts package/dist/cdn.min.js to the ignored static/js/alpine.min.js; script/test runs it, so make test, make check, the pre-commit hook and the Dockerfile get the file with no network access, and make build, run and dev run it too.

Removed: script/fetch-assets, its Dockerfile step, static/vendor.sha256 and static/vendor_test.go. The README describes the new flow.

Model: opus-5-5

.dockerignore

@@ -3,10 +3,8 @@
 # stage of the Dockerfile.
 .git/
 bin/
-# Third-party browser assets are fetched and hash-verified inside the build by
-# script/fetch-assets. Excluding any host copy keeps a developer's working tree
-# from supplying the bytes that get shipped. The script and its
-# static/vendor.sha256 manifest stay in the context.
+# Extracted from 3p/ by `make assets` inside the build; a host copy is not
+# needed. The tarball in 3p/ must stay in the context.
 static/js/alpine.min.js
 *.md
 LICENSE