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

test/test_blink_unit.cpp

@@ -0,0 +1,132 @@
+#include <gtest/gtest.h>
+#include <gmock/gmock.h>
+
+#include "hal.h"
+#include "mock_hal.h"
+#include "blink_app.h"
+
+using ::testing::_;
+using ::testing::AnyNumber;
+using ::testing::Return;
+using ::testing::InSequence;
+using ::testing::HasSubstr;
+
+// ============================================================================
+// Unit Tests -- verify exact HAL interactions
+// ============================================================================
+
+class BlinkAppUnitTest : public ::testing::Test {
+protected:
+    void SetUp() override {
+        // Allow timing and serial calls by default so tests can focus
+        // on the specific interactions they care about.
+        ON_CALL(mock_, millis()).WillByDefault(Return(0));
+        ON_CALL(mock_, digitalRead(_)).WillByDefault(Return(HIGH));
+        EXPECT_CALL(mock_, serialBegin(_)).Times(AnyNumber());
+        EXPECT_CALL(mock_, serialPrintln(_)).Times(AnyNumber());
+        EXPECT_CALL(mock_, millis()).Times(AnyNumber());
+    }
+
+    ::testing::NiceMock<MockHal> mock_;
+};
+
+TEST_F(BlinkAppUnitTest, BeginConfiguresPins) {
+    BlinkApp app(&mock_, 13, 2);
+
+    EXPECT_CALL(mock_, pinMode(13, OUTPUT)).Times(1);
+    EXPECT_CALL(mock_, pinMode(2, INPUT_PULLUP)).Times(1);
+    EXPECT_CALL(mock_, serialBegin(115200)).Times(1);
+
+    app.begin();
+}
+
+TEST_F(BlinkAppUnitTest, StartsInSlowMode) {
+    BlinkApp app(&mock_);
+    app.begin();
+
+    EXPECT_FALSE(app.fastMode());
+    EXPECT_EQ(app.interval(), BlinkApp::SLOW_INTERVAL_MS);
+}
+
+TEST_F(BlinkAppUnitTest, TogglesLedAfterInterval) {
+    BlinkApp app(&mock_);
+
+    // begin() at t=0
+    ON_CALL(mock_, millis()).WillByDefault(Return(0));
+    app.begin();
+
+    // update() at t=500 should toggle LED
+    ON_CALL(mock_, millis()).WillByDefault(Return(500));
+    EXPECT_CALL(mock_, digitalWrite(13, _)).Times(1);
+    app.update();
+}
+
+TEST_F(BlinkAppUnitTest, DoesNotToggleBeforeInterval) {
+    BlinkApp app(&mock_);
+
+    ON_CALL(mock_, millis()).WillByDefault(Return(0));
+    app.begin();
+
+    // update() at t=499 -- not enough time, no toggle
+    ON_CALL(mock_, millis()).WillByDefault(Return(499));
+    EXPECT_CALL(mock_, digitalWrite(_, _)).Times(0);
+    app.update();
+}
+
+TEST_F(BlinkAppUnitTest, ButtonPressSwitchesToFastMode) {
+    BlinkApp app(&mock_, 13, 2);
+
+    ON_CALL(mock_, millis()).WillByDefault(Return(0));
+    ON_CALL(mock_, digitalRead(2)).WillByDefault(Return(HIGH));
+    app.begin();
+
+    // Simulate button press (falling edge: HIGH -> LOW)
+    ON_CALL(mock_, digitalRead(2)).WillByDefault(Return(LOW));
+    EXPECT_CALL(mock_, serialPrintln(HasSubstr("FAST"))).Times(1);
+    app.update();
+
+    EXPECT_TRUE(app.fastMode());
+    EXPECT_EQ(app.interval(), BlinkApp::FAST_INTERVAL_MS);
+}
+
+TEST_F(BlinkAppUnitTest, SecondButtonPressReturnsToSlowMode) {
+    BlinkApp app(&mock_, 13, 2);
+
+    ON_CALL(mock_, millis()).WillByDefault(Return(0));
+    ON_CALL(mock_, digitalRead(2)).WillByDefault(Return(HIGH));
+    app.begin();
+
+    // First press: fast mode
+    ON_CALL(mock_, digitalRead(2)).WillByDefault(Return(LOW));
+    app.update();
+    EXPECT_TRUE(app.fastMode());
+
+    // Release
+    ON_CALL(mock_, digitalRead(2)).WillByDefault(Return(HIGH));
+    app.update();
+
+    // Second press: back to slow
+    ON_CALL(mock_, digitalRead(2)).WillByDefault(Return(LOW));
+    EXPECT_CALL(mock_, serialPrintln(HasSubstr("SLOW"))).Times(1);
+    app.update();
+
+    EXPECT_FALSE(app.fastMode());
+}
+
+TEST_F(BlinkAppUnitTest, HoldingButtonDoesNotRepeatToggle) {
+    BlinkApp app(&mock_, 13, 2);
+
+    ON_CALL(mock_, millis()).WillByDefault(Return(0));
+    ON_CALL(mock_, digitalRead(2)).WillByDefault(Return(HIGH));
+    app.begin();
+
+    // Press and hold
+    ON_CALL(mock_, digitalRead(2)).WillByDefault(Return(LOW));
+    app.update();
+    EXPECT_TRUE(app.fastMode());
+
+    // Still held -- should NOT toggle again
+    app.update();
+    app.update();
+    EXPECT_TRUE(app.fastMode());  // still fast, not toggled back
+}