Commit yarn 4 in the tree and install with --immutable (closes #149)

Follows sneak's ruling at
#149 (comment). This
repository now carries yarn 4.18.1 under .yarn/releases, and whichever
yarn the distribution packages hands the work to it. yarn.lock is
converted once, and script/bootstrap installs with --immutable instead
of stopping on yarn's major version. Node on Alpine is nodejs-current.
Every Dockerfile stage that needs yarn also copies .yarnrc.yml and
.yarn/releases/. The policies and both checklists say so, with the
steps for converting and for a new repository.

Model: opus-5-5

script/bootstrap

@@ -4,8 +4,10 @@
 # installed tools are skipped. Base tools, node and yarn come from nix,
 # apt, brew, or apk (detected in that order), with no version and no
 # hash, because the distribution is trusted. It installs node, yarn and
-# the node packages only when yarn.lock is in the tree. Assumes nothing
-# is present.
+# the node packages only when yarn.lock is in the tree. Whichever yarn
+# the distribution packages hands the work to the release file that
+# yarnPath in .yarnrc.yml names, so git pins the yarn that runs. Assumes
+# nothing is present.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
@@ -78,21 +80,12 @@ main() {
 
     # A Dockerfile stage that needs no yarn copies no yarn.lock.
     if [ -f yarn.lock ]; then
-        if missing node; then pkg_install nodejs nodejs node nodejs; fi
+        if missing node; then pkg_install nodejs nodejs node nodejs-current; fi
         if missing yarn && missing yarnpkg; then
             pkg_install yarn yarnpkg yarn yarn
         fi
 
-        # Debian 13 and Ubuntu 26.04 package yarn 4, which refuses a yarn 1
-        # yarn.lock, or rewrites it in its own format.
-        yarn_version="$(run_yarn --version)"
-        if [ "${yarn_version%%.*}" != "1" ]; then
-            echo "bootstrap: yarn $yarn_version refuses this yarn 1 yarn.lock;" \
-                "use a distribution that packages yarn 1" >&2
-            exit 1
-        fi
-
-        run_yarn install --frozen-lockfile
+        run_yarn install --immutable
     fi
 
     echo "bootstrap complete"