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

exercises/08_SystemStartup/lib/system_startup/SystemStartup.h

@@ -0,0 +1,46 @@
+#pragma once
+
+#include <Arduino.h>
+#include "StartupSdManager.h"
+
+// Convenience alias so sketches can use System.println(...) style logging.
+// Arduino exposes Serial, not System, so map System -> Serial.
+#ifndef System
+#define System Serial
+#endif
+
+enum class SystemEvent : uint8_t {
+  BOOTING = 0,
+  SD_MISSING,
+  SD_READY,
+  SD_REMOVED
+};
+
+using SystemEventCallback = void (*)(SystemEvent event, const char* message);
+
+struct SystemStartupConfig {
+  uint32_t serialDelayMs = 5000;
+  SdWatcherConfig sd{};
+};
+
+class SystemStartup {
+ public:
+  explicit SystemStartup(Print& serial = Serial);
+
+  bool begin(const SystemStartupConfig& cfg = SystemStartupConfig{}, SystemEventCallback callback = nullptr);
+  void update();
+
+  bool isSdMounted() const { return sd_.isMounted(); }
+  StartupSdManager& sdManager() { return sd_; }
+
+ private:
+  static void onSdEventThunk(SdEvent event, const char* message);
+  void onSdEvent(SdEvent event, const char* message);
+  void emit(SystemEvent event, const char* message);
+  void oledShow3(const char* l1, const char* l2 = nullptr, const char* l3 = nullptr);
+
+  Print& serial_;
+  SystemStartupConfig cfg_{};
+  SystemEventCallback callback_ = nullptr;
+  StartupSdManager sd_;
+};