chore(ci): temporary memory probe for the app build heap ceiling

Author: TheodoreSpeaks

.github/workflows/memprobe.yml

@@ -0,0 +1,70 @@
+name: Memory Probe
+
+# Throwaway diagnostic. Builds only the app.Dockerfile builder stage (the stage
+# that OOM-killed) on a GitHub-hosted runner across heap ceilings, sampling peak
+# memory. 8192 is the control: it must fail, or a pass at 4096 proves nothing.
+# Delete this file and the ci-memtest branch once the number is settled.
+
+on:
+  push:
+    branches: [ci-memtest]
+
+permissions:
+  contents: read
+
+jobs:
+  probe:
+    name: Build @ ${{ matrix.heap }} MB
+    runs-on: ubuntu-latest
+    timeout-minutes: 45
+    strategy:
+      fail-fast: false
+      matrix:
+        heap: [8192, 6144, 4096]
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6
+
+      - name: Runner specs
+        run: |
+          echo "nproc: $(nproc)"
+          free -h
+          echo "MemTotal: $(awk '/MemTotal/{printf "%.2f GiB\n", $2/1048576}' /proc/meminfo)"
+
+      - name: Start memory sampler
+        run: |
+          nohup sh -c 'while true; do
+            awk "/MemTotal/{t=\$2} /MemAvailable/{a=\$2} END{printf \"%d\n\", (t-a)/1024}" /proc/meminfo
+            sleep 3
+          done' > /tmp/mem-samples.log 2>/dev/null &
+          echo "sampler_pid=$!" >> $GITHUB_ENV
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@bb05f3f5519dd87d3ba754cc423b652a5edd6d2c # v4
+
+      - name: Build builder stage
+        id: build
+        continue-on-error: true
+        uses: docker/build-push-action@53b7df96c91f9c12dcc8a07bcb9ccacbed38856a # v7
+        with:
+          context: .
+          file: ./docker/app.Dockerfile
+          target: builder
+          platforms: linux/amd64
+          push: false
+          build-args: NEXT_BUILD_HEAP_MB=${{ matrix.heap }}
+          provenance: false
+          sbom: false
+
+      - name: Report
+        run: |
+          kill "${sampler_pid}" 2>/dev/null || true
+          PEAK=$(sort -n /tmp/mem-samples.log | tail -1)
+          echo "heap_ceiling_mb=${{ matrix.heap }}"
+          echo "build_outcome=${{ steps.build.outcome }}"
+          echo "peak_used_mb=${PEAK}"
+          {
+            echo "| heap MB | outcome | peak used MB |"
+            echo "|---|---|---|"
+            echo "| ${{ matrix.heap }} | ${{ steps.build.outcome }} | ${PEAK} |"
+          } >> $GITHUB_STEP_SUMMARY