Install the pinned node when the installed one is another major version (closes #118)

script/bootstrap used whatever node was installed, so on the CI runner
image, which ships node 24, the pinned yarn 1.22.22 printed the
deprecation warning DEP0169 on every bootstrap. It now uses the
installed node only when its major version is the pinned one, and
otherwise installs the pinned node with nvm, as it already did when node
was missing, with yarn and the packages under it. It prints the node
version it uses. Only the major is compared because the Dockerfile
stages use a node 22 alpine image that nvm cannot replace. script/fmt
and script/fmt-check carry the same test to pick their yarn: the yarn on
PATH, or yarn under the pinned node loaded through nvm.

Model: opus-5-5

script/bootstrap

@@ -2,10 +2,12 @@
 # script/bootstrap: install all dependencies needed to build and develop
 # this repo. Idempotent: every install is guarded by a check so already
 # installed tools are skipped. Base tooling comes from nix, apt, brew,
-# or apk (detected in that order); assumes nothing is present. Node is
-# used directly if installed; otherwise it is installed at a pinned
-# version via nvm (installing nvm itself first, from a hash-verified
-# release archive, never curl | sh).
+# or apk (detected in that order); assumes nothing is present. The
+# installed node is used only when its major version is the pinned one;
+# otherwise (node missing, or another major) the pinned version is
+# installed via nvm (installing nvm itself first, from a hash-verified
+# release archive, never curl | sh), and yarn and the packages are
+# installed under it.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
@@ -100,31 +102,52 @@ ensure_nvm() {
     rm -rf "$tmp"
 }
 
+# True when the node on PATH has the pinned major version. Only the
+# major is compared: the Dockerfile stages start from a node 22 alpine
+# image whose exact version is not the pin, and nvm cannot install a
+# prebuilt node on alpine. Another major is not used: the pinned yarn 1
+# prints a deprecation warning under node 24. script/fmt and
+# script/fmt-check carry this function unchanged, to run the yarn
+# installed here.
+node_is_pinned_major() {
+    if ! command -v node >/dev/null 2>&1; then return 1; fi
+    installed="$(node --version)"
+    installed="${installed#v}"
+    [ "${installed%%.*}" = "${NODE_VERSION%%.*}" ]
+}
+
 ensure_node() {
-    if ! missing node; then return 0; fi
+    if node_is_pinned_major; then
+        echo "bootstrap: using node $(node --version)"
+        return 0
+    fi
     ensure_nvm
     nvm_sh "nvm install $NODE_VERSION"
+    nvm_sh "nvm use $NODE_VERSION >/dev/null && \
+        echo \"bootstrap: using node \$(node --version)\""
 }
 
 ensure_yarn() {
+    if ! node_is_pinned_major; then
+        nvm_sh "nvm use $NODE_VERSION >/dev/null && corepack enable && \
+            corepack prepare yarn@$YARN_VERSION --activate"
+        return 0
+    fi
     if ! missing yarn; then return 0; fi
     if ! missing corepack; then
         corepack enable
         corepack prepare "yarn@$YARN_VERSION" --activate
-    elif [ -s "$HOME/.nvm/nvm.sh" ]; then
-        nvm_sh "nvm use $NODE_VERSION >/dev/null && corepack enable && \
-            corepack prepare yarn@$YARN_VERSION --activate"
     else
         npm install -g "yarn@$YARN_VERSION"
     fi
 }
 
 install_js_deps() {
-    if missing yarn && [ -s "$HOME/.nvm/nvm.sh" ]; then
+    if node_is_pinned_major; then
+        yarn install --frozen-lockfile
+    else
         nvm_sh "nvm use $NODE_VERSION >/dev/null && cd \"$ROOT\" && \
             yarn install --frozen-lockfile"
-    else
-        yarn install --frozen-lockfile
     fi
 }