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

README.md

@@ -0,0 +1,79 @@
+# SparkFun RedBoard Projects
+
+Arduino sketches for SparkFun RedBoard, built and uploaded via `arduino-build.sh`.
+
+## Prerequisites
+
+- [arduino-cli](https://arduino.github.io/arduino-cli/)
+- Run `./arduino-build.sh --setup` once after installing
+
+## Quick Start
+
+```bash
+./arduino-build.sh --devices           # find your board
+./arduino-build.sh --monitor ./blink   # build, upload, serial console
+```
+
+## Project Layout
+
+```
+sparkfun/
+    arduino-build.sh        Build/upload/monitor tool (run --help)
+    blink/
+        blink.ino           LED blink + serial hello world
+```
+
+## Two-Terminal Workflow
+
+```bash
+# Terminal 1: persistent serial monitor
+./arduino-build.sh --watch
+
+# Terminal 2: edit, build, upload (monitor reconnects automatically)
+./arduino-build.sh ./blink
+```
+
+## Adding a New Sketch
+
+```bash
+mkdir my_sketch
+# Create my_sketch/my_sketch.ino (filename must match directory)
+./arduino-build.sh --verify ./my_sketch   # compile check
+./arduino-build.sh ./my_sketch            # build + upload
+```
+
+## Troubleshooting
+
+### Ubuntu: Board not detected (CH340 / RedBoard)
+
+On Ubuntu, `brltty` (a braille display daemon) may claim the CH340 USB-serial
+interface before the `ch341` kernel driver can bind to it. The board will appear
+in `lsusb` but no `/dev/ttyUSB*` device is created.
+
+Symptoms in `dmesg`:
+
+```
+usb 1-3.1.3: usbfs: interface 0 claimed by ch341 while 'brltty' sets config #1
+ch341-uart ttyUSB0: ch341-uart converter now disconnected from ttyUSB0
+```
+
+If you are not using a braille display, remove `brltty` entirely:
+
+```bash
+sudo systemctl stop brltty-udev
+sudo systemctl disable brltty-udev
+sudo systemctl stop brltty
+sudo systemctl disable brltty
+sudo apt remove brltty
+```
+
+Alternatively, to keep `brltty` installed but block it from claiming CH340
+devices, add a udev rule:
+
+```bash
+sudo tee /etc/udev/rules.d/99-ch340-no-brltty.rules << 'EOF'
+SUBSYSTEM=="usb", ATTRS{idVendor}=="1a86", ATTRS{idProduct}=="7523", ENV{BRLTTY_BRAILLE_DRIVER}="none"
+EOF
+sudo udevadm control --reload-rules
+sudo udevadm trigger
+```