ci: install npm via direct tarball (corepack doesn't replace PATH)

.github/workflows/release-please.yml

@@ -61,12 +61,51 @@ jobs:
       - name: Install a working npm (bundled npm in Node 22.22.2 is broken)
         # Node 22.22.2 ships with an npm install that is missing the
         # `promise-retry` dependency, so `npm install -g npm@latest` fails
-        # with MODULE_NOT_FOUND before it can upgrade itself. Use corepack
-        # to install a known-good npm version that supports OIDC trusted
-        # publishing.
+        # with MODULE_NOT_FOUND before it can upgrade itself. Corepack's
+        # `prepare --activate` stages a new npm in the corepack cache but
+        # does not actually replace the npm binary in the PATH that this
+        # shell sees (it prints the old version). Instead, download the
+        # [email protected] tarball directly and extract it over the broken
+        # bundled install. npm 11.6.0 supports OIDC trusted publishing.
+        #
+        # SECURITY: we verify the tarball against the sha512 integrity
+        # value published in the registry's package metadata before
+        # extracting it. Both requests go to registry.npmjs.org (the
+        # same origin we're about to publish to), so the trust model is
+        # unchanged, but this protects against in-transit tampering
+        # between the metadata fetch and the tarball fetch, and ensures
+        # we install the exact bytes the registry says 11.6.0 should be.
         run: |
-          corepack enable
-          corepack prepare [email protected] --activate
+          set -euo pipefail
+          NPM_VERSION=11.6.0
+          NODE_BIN=$(which node)
+          NODE_DIR=$(dirname $(dirname $NODE_BIN))
+          NPM_DIR="$NODE_DIR/lib/node_modules/npm"
+          TMP=$(mktemp -d)
+
+          EXPECTED_INTEGRITY=$(
+            curl -fsSL "https://registry.npmjs.org/npm/$NPM_VERSION" \
+            | python3 -c "import json, sys; print(json.load(sys.stdin)['dist']['integrity'])"
+          )
+          if [[ "$EXPECTED_INTEGRITY" != sha512-* ]]; then
+            echo "unexpected integrity format from registry: $EXPECTED_INTEGRITY" >&2
+            exit 1
+          fi
+
+          curl -fsSL "https://registry.npmjs.org/npm/-/npm-$NPM_VERSION.tgz" -o "$TMP/npm.tgz"
+
+          ACTUAL_INTEGRITY="sha512-$(openssl dgst -sha512 -binary "$TMP/npm.tgz" | base64 -w0)"
+          if [ "$ACTUAL_INTEGRITY" != "$EXPECTED_INTEGRITY" ]; then
+            echo "npm tarball integrity check failed" >&2
+            echo "  expected: $EXPECTED_INTEGRITY" >&2
+            echo "  actual:   $ACTUAL_INTEGRITY" >&2
+            exit 1
+          fi
+
+          tar -xzf "$TMP/npm.tgz" -C "$TMP"
+          sudo rm -rf "$NPM_DIR"
+          sudo mv "$TMP/package" "$NPM_DIR"
+          hash -r
           npm --version
 
       - name: Install dependencies