exercises: add Exercise 08 SystemStartup scaffold with early SPI deselect and SD/OLED startup orchestration

exercises/08_SystemStartup/src/main.cpp

@@ -0,0 +1,21 @@
+// 20260216 ChatGPT
+// $Id$
+// $HeadURL$
+
+#include <Arduino.h>
+#include "SystemStartup.h"
+
+static SystemStartup g_systemStartup(Serial);
+
+void setup() {
+  Serial.begin(115200);
+  System.println("Example 08: setup() called. About to call g_systemStartup.begin().");
+  g_systemStartup.begin();
+  System.println("After g_systemStartup() in setup().");
+}
+
+void loop() {
+  g_systemStartup.update();
+  System.println("Example 08 loop() called and after g_systemStartup.update().");
+  delay(1000);
+}