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/_docker.sh

@@ -0,0 +1,14 @@
+# Shared helper: run a command inside the roro9stack build container.
+IMAGE=roro9stack-build
+ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+
+docker build -q -t "$IMAGE" "$ROOT/docker" >/dev/null
+
+run_in_container() {
+    docker run --rm \
+        -u "$(id -u):$(id -g)" -e HOME=/tmp \
+        -v "$ROOT:/work" \
+        -v roro9stack-pio:/pio \
+        "${DOCKER_EXTRA[@]}" \
+        "$IMAGE" "$@"
+}