ci: install the pinned node from its tarball instead of trusting the runner image

.gitea/workflows/gates.yml

@@ -50,10 +50,12 @@ env:
   # Exact patch, not a floating "24" (milestone-14 ruling 12): the bundled npm
   # and the emitted bundle change under a floating major.
   NODE_VERSION: "24.19.0"
+  # node-v24.19.0-linux-x64.tar.gz, as nodejs.org's SHASUMS256.txt lists it.
+  NODE_TARBALL_SHA256: "f625d97cd707df4ff96254916fbc5ff014f09c09effe5a1e0ca8f6d41a8789d4"
   # The npm the cut runs and the npm CI runs must be one version: npm writes the
   # admin bundle whose bytes the release hashes are pinned to before CI ever
-  # rebuilds them. The runner image ships the npm that comes with the node
-  # above, so this pin is asserted, not installed.
+  # rebuilds them. The tarball above bundles this npm, so this pin is asserted,
+  # not installed.
   NPM_VERSION: "11.17.0"
   # There is deliberately no CI_VERSION literal here. Ruling 2 allows the
   # version to exist in the tag and in build.zig.zon and nowhere else, and
@@ -138,9 +140,17 @@ jobs:
     steps:
       - uses: actions/checkout@11d5960a326750d5838078e36cf38b85af677262 # v4.4.0
 
-      # The runner image ships this Node and npm on PATH; the assertion below is
-      # what pins them. setup-node is not used: it looks for Node in an empty
-      # tool cache and downloads it again on every run.
+      # Installed from the official tarball, pinned by its digest: the runner
+      # image's Node moves with the image tag, and a rebuilt image on a newer
+      # Node failed the assertion below on 2026-10-22.
+      - name: Install the pinned Node
+        run: |
+          curl -fsSLo "${RUNNER_TEMP:?}/node.tar.gz" "https://nodejs.org/dist/v${NODE_VERSION:?}/node-v${NODE_VERSION:?}-linux-x64.tar.gz"
+          echo "${NODE_TARBALL_SHA256:?}  ${RUNNER_TEMP:?}/node.tar.gz" | sha256sum -c -
+          mkdir "${RUNNER_TEMP:?}/node"
+          tar -xzf "${RUNNER_TEMP:?}/node.tar.gz" -C "${RUNNER_TEMP:?}/node" --strip-components=1
+          echo "${RUNNER_TEMP:?}/node/bin" >> "${GITHUB_PATH:?}"
+
       - name: Cache npm downloads
         uses: actions/cache@0057852bfaa89a56745cba8c7296529d2fc39830 # v4.3.0
         with: