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

scripts/flash.sh

@@ -0,0 +1,9 @@
+#!/usr/bin/env bash
+# Flash the firmware over USB, then open the serial monitor.
+# Usage: scripts/flash.sh [/dev/ttyACM0]
+set -euo pipefail
+PORT="${1:-/dev/ttyACM0}"
+source "$(dirname "$0")/_docker.sh"
+DOCKER_EXTRA=(--device "$PORT" --group-add "$(stat -c %g "$PORT")" -it)
+
+run_in_container bash -c "git config --global --add safe.directory /work && pio run -e cardputer-adv -t upload --upload-port $PORT && pio device monitor -p $PORT -b 115200"