ci(build): switch Next.js production build to Turbopack (1.9x faster)

.github/workflows/ci.yml

@@ -519,14 +519,21 @@ jobs:
           cache: npm
       - uses: ./.github/actions/npm-ci-retry
       - run: npm run check:node-runtime
-      - name: Cache Next.js build cache
-        uses: actions/cache@55cc8345863c7cc4c66a329aec7e433d2d1c52a9
-        with:
-          path: .build/next/cache
-          key: nextjs-${{ runner.os }}-node-${{ env.CI_NODE_VERSION }}-${{ hashFiles('package-lock.json') }}-${{ hashFiles('src/**/*', 'open-sse/**/*', 'db/**/*', 'next.config.mjs', 'tsconfig*.json', 'postcss.config.*', 'tailwind.config.*') }}
-          restore-keys: |
-            nextjs-${{ runner.os }}-node-${{ env.CI_NODE_VERSION }}-${{ hashFiles('package-lock.json') }}-
+      # NOTE: the webpack `.build/next/cache` actions/cache step was removed with the
+      # Turbopack switch below — Turbopack does not read/write the webpack cache dir
+      # (its persistent FS cache is still experimental and intentionally NOT enabled),
+      # so restoring the old ~0.5 GB webpack cache would only waste download time.
+      # Rolling back to webpack = revert this commit (cache step comes back with it).
+      #
+      # Turbopack production build (Next 16, stable): benchmarked 1.9× faster than the
+      # webpack pass (9min0s vs 17min15s on a 32-core box; multi-core Rust vs webpack's
+      # single-threaded compile). Standalone output smoke-validated (server boots,
+      # /api/monitoring/health 200). Downstream jobs (e2e ×9, package-artifact,
+      # electron-package-smoke) consume this artifact, so a green run here validates
+      # the Turbopack artifact end-to-end.
       - run: npm run build
+        env:
+          OMNIROUTE_USE_TURBOPACK: "1"
       - name: Archive Next.js build for downstream jobs
         # Use tar so the archive preserves paths relative to CWD (.build/next/...).
         # upload-artifact path-stripping is ambiguous when exclude patterns are used;