Add Node.js to test containers for sexp.js parity tests

Node 20 from Debian packages — needed to run test_sexp_js.py which
verifies JS renderer output matches Python renderer output.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

test/Dockerfile.unit

@@ -8,6 +8,10 @@ ENV PYTHONDONTWRITEBYTECODE=1 \
 
 WORKDIR /app
 
+# Node.js for sexp.js parity tests
+RUN apt-get update && apt-get install -y --no-install-recommends nodejs && \
+    rm -rf /var/lib/apt/lists/*
+
 # Install shared deps (includes pytest, pytest-asyncio)
 COPY shared/requirements.txt ./requirements-shared.txt
 RUN pip install --no-cache-dir -r requirements-shared.txt pytest-watch