Add AtomS3R button color demo

.gitignore

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+.pio/
+.vscode/.browse.c_cpp.db*
+.vscode/c_cpp_properties.json
+.vscode/launch.json
+.vscode/ipch/
+

README.md

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+# M5Stack AtomS3R Color Button
+
+A simple PlatformIO program for the M5Stack AtomS3R.
+
+Each press of the screen/button cycles through red, green, blue, yellow,
+magenta, cyan, white, and black. The selected color name is drawn in the
+center and also printed to the serial monitor at 115200 baud.
+
+## Build
+
+```powershell
+pio run
+```
+
+## Upload
+
+Connect the AtomS3R over USB, then run:
+
+```powershell
+pio run --target upload
+```
+
+If normal upload mode is not detected, hold the reset button for about two
+seconds until the internal green LED lights, release it, and retry the upload.

platformio.ini

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+[env:m5stack-atoms3r]
+platform = espressif32
+board = m5stack-atoms3
+framework = arduino
+monitor_speed = 115200
+lib_deps =
+  m5stack/M5Unified@^0.2.19
+

src/main.cpp

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+#include <Arduino.h>
+#include <M5Unified.h>
+
+namespace {
+
+struct ScreenColor {
+  uint32_t value;
+  const char* name;
+};
+
+constexpr ScreenColor COLORS[] = {
+    {TFT_RED, "RED"},
+    {TFT_GREEN, "GREEN"},
+    {TFT_BLUE, "BLUE"},
+    {TFT_YELLOW, "YELLOW"},
+    {TFT_MAGENTA, "MAGENTA"},
+    {TFT_CYAN, "CYAN"},
+    {TFT_WHITE, "WHITE"},
+    {TFT_BLACK, "BLACK"},
+};
+
+constexpr size_t COLOR_COUNT = sizeof(COLORS) / sizeof(COLORS[0]);
+size_t colorIndex = 0;
+
+void showColor() {
+  const ScreenColor& color = COLORS[colorIndex];
+  M5.Display.fillScreen(color.value);
+
+  // Choose contrasting text so the color name remains readable.
+  M5.Display.setTextColor(color.value == TFT_WHITE || color.value == TFT_YELLOW
+                              ? TFT_BLACK
+                              : TFT_WHITE);
+  M5.Display.setTextDatum(middle_center);
+  M5.Display.setTextSize(2);
+  M5.Display.drawString(color.name,
+                        M5.Display.width() / 2,
+                        M5.Display.height() / 2);
+
+  Serial.printf("Color: %s\n", color.name);
+}
+
+}  // namespace
+
+void setup() {
+  auto config = M5.config();
+  M5.begin(config);
+
+  Serial.begin(115200);
+  M5.Display.setRotation(0);
+  showColor();
+}
+
+void loop() {
+  M5.update();
+
+  // The AtomS3R's screen is also its physical button.
+  if (M5.BtnA.wasPressed()) {
+    colorIndex = (colorIndex + 1) % COLOR_COUNT;
+    showColor();
+  }
+
+  delay(10);
+}
+