feat: add web playground with Docker sandbox

- playground/backend/main.go — Go HTTP server with /compile endpoint
- playground/frontend/index.html — Monaco Editor with 8 code examples
- playground/sandbox/Dockerfile — isolated Docker container with buxc2
- playground/docker-compose.yml — orchestrates backend + sandbox
- playground/Makefile — build and run targets
- playground/README.md — deployment guide for VPS with nginx + SSL

playground/sandbox/Dockerfile

@@ -0,0 +1,33 @@
+# Bux Playground Sandbox
+# Minimal container with Bux compiler + C toolchain
+FROM debian:bookworm-slim
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    gcc \
+    clang \
+    libc6-dev \
+    libssl-dev \
+    make \
+    ca-certificates \
+    && rm -rf /var/lib/apt/lists/*
+
+# Create sandbox user (no root)
+RUN useradd -m -s /bin/bash sandbox
+
+# Copy Bux compiler binary
+COPY buxc2 /usr/local/bin/buxc2
+RUN chmod +x /usr/local/bin/buxc2
+
+# Copy stdlib and runtime
+COPY lib /usr/local/share/bux/lib
+COPY rt /usr/local/share/bux/rt
+
+# Set up working directory
+WORKDIR /tmp/bux-run
+RUN chown sandbox:sandbox /tmp/bux-run
+
+# Security: drop privileges, limit resources
+USER sandbox
+
+# Default command (backend will override)
+CMD ["/bin/bash"]