ci(build): switch Next.js production build to Turbopack (1.9x faster) (#6273)

Next 16 ships Turbopack as the stable production bundler. Benchmarked on a
32-core box against the same tree: webpack 1035s -> Turbopack 539s (1.92x).
The build script already supports the switch via OMNIROUTE_USE_TURBOPACK=1
(scripts/build/build-next-isolated.mjs) and next.config.mjs already mirrors
the resolveAlias stubs in the turbopack block, so this only flips the CI env.

The webpack .build/next/cache actions/cache step is removed in the same
commit: Turbopack does not use the webpack cache dir (its persistent FS
cache is experimental and NOT enabled), so restoring ~0.5 GB per run would
be pure wasted download. Revert restores webpack + its cache together.

Validation: standalone output smoke-tested (server.js boots, health 200,
dashboard 307); the 428 build warnings are the known benign 'overly broad
file pattern' static-analysis notices for dynamic fs usage (covered at
runtime by outputFileTracingIncludes). Downstream e2e x9, package-artifact
and electron-package-smoke consume this artifact, so a green CI run here
validates the Turbopack artifact end-to-end. nightly-compat and npm-publish
stay on webpack until this PR proves out.

.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;