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/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