Arduino CLI build system with HAL-based test architecture

Build and upload tool (arduino-build.sh):
- Compile, upload, and monitor via arduino-cli
- Device discovery with USB ID identification (--devices)
- Persistent reconnecting serial monitor (--watch)
- Split compile/upload workflow (--verify, --upload-only)
- First-time setup wizard (--setup)
- Comprehensive --help with troubleshooting and RedBoard specs

Testable application architecture:
- Hardware abstraction layer (lib/hal/) decouples logic from Arduino API
- Google Mock HAL for unit tests (exact call verification)
- Simulated HAL for system tests (GPIO state, virtual clock, I2C devices)
- Example I2C temperature sensor simulator (TMP102)
- Host-side test suite via CMake + Google Test (21 tests)

Example sketch:
- blink/ -- LED blink with button-controlled speed, wired through HAL

lib/app/blink_app.h

@@ -0,0 +1,88 @@
+#ifndef BLINK_APP_H
+#define BLINK_APP_H
+
+#include <hal.h>
+
+/*
+ * BlinkApp -- Testable blink logic, decoupled from hardware.
+ *
+ * Blinks an LED and reads a button. When the button is pressed,
+ * the blink rate doubles (toggles between normal and fast mode).
+ *
+ * All hardware access goes through the injected Hal pointer. This
+ * class has no dependency on Arduino.h and compiles on any host.
+ */
+class BlinkApp {
+public:
+    static constexpr uint8_t DEFAULT_LED_PIN    = LED_BUILTIN;  // pin 13
+    static constexpr uint8_t DEFAULT_BUTTON_PIN = 2;
+    static constexpr unsigned long SLOW_INTERVAL_MS = 500;
+    static constexpr unsigned long FAST_INTERVAL_MS = 125;
+
+    BlinkApp(Hal* hal,
+             uint8_t led_pin = DEFAULT_LED_PIN,
+             uint8_t button_pin = DEFAULT_BUTTON_PIN)
+        : hal_(hal)
+        , led_pin_(led_pin)
+        , button_pin_(button_pin)
+        , led_state_(LOW)
+        , fast_mode_(false)
+        , last_toggle_ms_(0)
+        , last_button_state_(HIGH)  // pulled up, so HIGH = not pressed
+    {}
+
+    // Call once from setup()
+    void begin() {
+        hal_->pinMode(led_pin_, OUTPUT);
+        hal_->pinMode(button_pin_, INPUT_PULLUP);
+        hal_->serialBegin(115200);
+        hal_->serialPrintln("BlinkApp started");
+        last_toggle_ms_ = hal_->millis();
+    }
+
+    // Call repeatedly from loop()
+    void update() {
+        handleButton();
+        handleBlink();
+    }
+
+    // -- Accessors for testing ----------------------------------------------
+    bool ledState() const       { return led_state_ == HIGH; }
+    bool fastMode() const       { return fast_mode_; }
+    unsigned long interval() const {
+        return fast_mode_ ? FAST_INTERVAL_MS : SLOW_INTERVAL_MS;
+    }
+
+private:
+    void handleButton() {
+        uint8_t reading = hal_->digitalRead(button_pin_);
+
+        // Detect falling edge (HIGH -> LOW = button press with INPUT_PULLUP)
+        if (last_button_state_ == HIGH && reading == LOW) {
+            fast_mode_ = !fast_mode_;
+            hal_->serialPrintln(fast_mode_ ? "FAST" : "SLOW");
+        }
+        last_button_state_ = reading;
+    }
+
+    void handleBlink() {
+        unsigned long now = hal_->millis();
+        unsigned long target = fast_mode_ ? FAST_INTERVAL_MS : SLOW_INTERVAL_MS;
+
+        if (now - last_toggle_ms_ >= target) {
+            led_state_ = (led_state_ == HIGH) ? LOW : HIGH;
+            hal_->digitalWrite(led_pin_, led_state_);
+            last_toggle_ms_ = now;
+        }
+    }
+
+    Hal* hal_;
+    uint8_t led_pin_;
+    uint8_t button_pin_;
+    uint8_t led_state_;
+    bool fast_mode_;
+    unsigned long last_toggle_ms_;
+    uint8_t last_button_state_;
+};
+
+#endif // BLINK_APP_H

lib/hal/hal.h

@@ -0,0 +1,67 @@
+#ifndef HAL_H
+#define HAL_H
+
+#include <stdint.h>
+
+// Pin modes (match Arduino constants)
+#ifndef INPUT
+#define INPUT  0x0
+#define OUTPUT 0x1
+#define INPUT_PULLUP 0x2
+#endif
+
+#ifndef LOW
+#define LOW  0x0
+#define HIGH 0x1
+#endif
+
+// LED_BUILTIN for host builds
+#ifndef LED_BUILTIN
+#define LED_BUILTIN 13
+#endif
+
+/*
+ * Hardware Abstraction Layer
+ *
+ * Abstract interface over GPIO, timing, serial, and I2C. Sketch logic
+ * depends on this interface only -- never on Arduino.h directly.
+ *
+ * Two implementations:
+ *   hal_arduino.h  -- real hardware (included by .ino files)
+ *   mock_hal.h     -- Google Mock (included by test files)
+ */
+class Hal {
+public:
+    virtual ~Hal() = default;
+
+    // -- GPIO ---------------------------------------------------------------
+    virtual void pinMode(uint8_t pin, uint8_t mode) = 0;
+    virtual void digitalWrite(uint8_t pin, uint8_t value) = 0;
+    virtual uint8_t digitalRead(uint8_t pin) = 0;
+    virtual int analogRead(uint8_t pin) = 0;
+    virtual void analogWrite(uint8_t pin, int value) = 0;
+
+    // -- Timing -------------------------------------------------------------
+    virtual unsigned long millis() = 0;
+    virtual unsigned long micros() = 0;
+    virtual void delay(unsigned long ms) = 0;
+    virtual void delayMicroseconds(unsigned long us) = 0;
+
+    // -- Serial -------------------------------------------------------------
+    virtual void serialBegin(unsigned long baud) = 0;
+    virtual void serialPrint(const char* msg) = 0;
+    virtual void serialPrintln(const char* msg) = 0;
+    virtual int serialAvailable() = 0;
+    virtual int serialRead() = 0;
+
+    // -- I2C ----------------------------------------------------------------
+    virtual void i2cBegin() = 0;
+    virtual void i2cBeginTransmission(uint8_t addr) = 0;
+    virtual size_t i2cWrite(uint8_t data) = 0;
+    virtual uint8_t i2cEndTransmission() = 0;
+    virtual uint8_t i2cRequestFrom(uint8_t addr, uint8_t count) = 0;
+    virtual int i2cAvailable() = 0;
+    virtual int i2cRead() = 0;
+};
+
+#endif // HAL_H

lib/hal/hal_arduino.h

@@ -0,0 +1,93 @@
+#ifndef HAL_ARDUINO_H
+#define HAL_ARDUINO_H
+
+/*
+ * Real hardware implementation of the HAL.
+ *
+ * This file includes Arduino.h and Wire.h, so it can only be compiled
+ * by avr-gcc (via arduino-cli). It is included by .ino files only.
+ *
+ * Every method is a trivial passthrough to the real Arduino function.
+ * The point is not to add logic here -- it is to keep Arduino.h out
+ * of your application code so that code can compile on the host.
+ */
+
+#include <Arduino.h>
+#include <Wire.h>
+#include <hal.h>
+
+class ArduinoHal : public Hal {
+public:
+    // -- GPIO ---------------------------------------------------------------
+    void pinMode(uint8_t pin, uint8_t mode) override {
+        ::pinMode(pin, mode);
+    }
+    void digitalWrite(uint8_t pin, uint8_t value) override {
+        ::digitalWrite(pin, value);
+    }
+    uint8_t digitalRead(uint8_t pin) override {
+        return ::digitalRead(pin);
+    }
+    int analogRead(uint8_t pin) override {
+        return ::analogRead(pin);
+    }
+    void analogWrite(uint8_t pin, int value) override {
+        ::analogWrite(pin, value);
+    }
+
+    // -- Timing -------------------------------------------------------------
+    unsigned long millis() override {
+        return ::millis();
+    }
+    unsigned long micros() override {
+        return ::micros();
+    }
+    void delay(unsigned long ms) override {
+        ::delay(ms);
+    }
+    void delayMicroseconds(unsigned long us) override {
+        ::delayMicroseconds(us);
+    }
+
+    // -- Serial -------------------------------------------------------------
+    void serialBegin(unsigned long baud) override {
+        Serial.begin(baud);
+    }
+    void serialPrint(const char* msg) override {
+        Serial.print(msg);
+    }
+    void serialPrintln(const char* msg) override {
+        Serial.println(msg);
+    }
+    int serialAvailable() override {
+        return Serial.available();
+    }
+    int serialRead() override {
+        return Serial.read();
+    }
+
+    // -- I2C ----------------------------------------------------------------
+    void i2cBegin() override {
+        Wire.begin();
+    }
+    void i2cBeginTransmission(uint8_t addr) override {
+        Wire.beginTransmission(addr);
+    }
+    size_t i2cWrite(uint8_t data) override {
+        return Wire.write(data);
+    }
+    uint8_t i2cEndTransmission() override {
+        return Wire.endTransmission();
+    }
+    uint8_t i2cRequestFrom(uint8_t addr, uint8_t count) override {
+        return Wire.requestFrom(addr, count);
+    }
+    int i2cAvailable() override {
+        return Wire.available();
+    }
+    int i2cRead() override {
+        return Wire.read();
+    }
+};
+
+#endif // HAL_ARDUINO_H