Scaffold PlatformIO project with Docker-based build, test and flash

- pioarduino platform 55.03.312 (Arduino-ESP32 3.3.x / ESP-IDF 5.5),
  board m5stack-stamps3, M5Cardputer 1.1.1
- native env with Unity for host-side tests
- scripts/ci.sh and scripts/flash.sh run PlatformIO in a container
- version injected from git describe
- boot stub shows name, version and pressed keys
- GPL-3.0 license

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EhqxQ49eCju4CzKYNjZzwT

docker/Dockerfile

@@ -0,0 +1,13 @@
+# Build/test/flash environment for roro9stack. No toolchain needed on the host.
+FROM python:3.12-slim
+
+RUN apt-get update \
+ && apt-get install -y --no-install-recommends git build-essential \
+ && rm -rf /var/lib/apt/lists/*
+
+RUN pip install --no-cache-dir platformio
+
+# Toolchains and libraries are cached in a named volume mounted here.
+RUN mkdir -p /pio && chmod 777 /pio
+ENV PLATFORMIO_CORE_DIR=/pio
+WORKDIR /work