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

script/bootstrap

@@ -4,9 +4,7 @@
 # installed tools are skipped. Base tooling comes from nix, apt, brew,
 # or apk (detected in that order); assumes NOTHING is present (not git,
 # make, or go). golangci-lint is deliberately not installed: linting runs
-# only in docker, via script/lint and Dockerfile.lint. Finishes by running
-# script/fetch-assets, which installs the hash-pinned third-party browser
-# assets the repo does not commit.
+# only in docker, via script/lint and Dockerfile.lint.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
@@ -69,11 +67,6 @@ main() {
 
     go mod download
 
-    # Third-party browser assets are not committed; fetch and verify them
-    # so a fresh clone can build and test.
-    if missing curl; then pkg_install curl curl curl curl; fi
-    "$ROOT/script/fetch-assets"
-
     echo "bootstrap complete"
 }