Introduce StateManager to persist state between power cycles (#6)

- add reset state menu option to return all settings back to default values.
- add reverse encoder menu option and save state
- make saving to EEPROM safer by wrapping put calls with noInterrupts()
- improve save state behavior by using a mutex flag and update check with debounce in main loop
- refactor gravity.h global const definitions to be static and more readable.
- improve usage of EncoderDir in ISR with pointer to instance and static isr() method.
- reduce u8g2 memory usage by using single page buffer

Reviewed-on: https://git.pinkduck.xyz/adam/libGravity/pulls/6
Co-authored-by: Adam Wonak <adam.wonak@gmail.com>
Co-committed-by: Adam Wonak <adam.wonak@gmail.com>

examples/Gravity/save_state.h

@@ -0,0 +1,65 @@
+#ifndef SAVE_STATE_H
+#define SAVE_STATE_H
+
+#include <Arduino.h>
+#include <gravity.h>
+
+// Forward-declare AppState to avoid circular dependencies.
+struct AppState;
+
+// Define the constants for the current firmware.
+const char CURRENT_SKETCH_NAME[] = "Gravity";
+const float CURRENT_SKETCH_VERSION = 0.2f;
+
+/**
+ * @brief Manages saving and loading of the application state to and from EEPROM.
+ */
+class StateManager {
+   public:
+    StateManager();
+
+    // Populate the AppState instance with values from EEPROM if they exist.
+    bool initialize(AppState& app);
+    // Reset AppState instance back to default values.
+    void reset(AppState& app);
+    // Call from main loop, check if state has changed and needs to be saved.
+    void update(const AppState& app);
+    // Indicate that state has changed and we should save.
+    void markDirty();
+
+   private:
+    // This struct holds the data that identifies the firmware version.
+    struct Metadata {
+        char sketchName[16];
+        byte version;
+    };
+    struct ChannelState {
+        byte base_clock_mod_index;
+        byte base_probability;
+        byte base_duty_cycle;
+        byte base_offset;
+        byte cv_source;       // Cast the CvSource enum to a byte for storage
+        byte cv_destination;  // Cast the CvDestination enum as a byte for storage
+    };
+    // This struct holds all the parameters we want to save.
+    struct EepromData {
+        int tempo;
+        bool encoder_reversed;
+        byte selected_param;
+        byte selected_channel;
+        byte selected_source;
+        ChannelState channel_data[Gravity::OUTPUT_COUNT];
+    };
+
+    void save(const AppState& app);
+
+    bool isDataValid();
+    void _save_worker(const AppState& app);
+    void _metadata_worker();
+
+    bool _isDirty;
+    unsigned long _lastChangeTime;
+    static const unsigned long SAVE_DELAY_MS = 2000;
+};
+
+#endif  // SAVE_STATE_H