Add mock Arduino for x86_64 host-side testing

Complete Arduino API mock (mock_arduino.h/cpp) enabling application
code to compile and run on PC without hardware. Includes MockSerial,
String class, GPIO/analog/timing/interrupt mocks with state tracking
and test control API.

- Arduino.h, Wire.h, SPI.h shims intercept includes in test builds
- System test template (test_system.cpp) using SimHal
- CMakeLists.txt builds mock_arduino as static lib, links both suites
- Root test.sh/test.bat with --unit/--system/--clean/--verbose flags
- test.bat auto-detects MSVC via vswhere + vcvarsall.bat
- Doctor reports nuanced compiler status (on PATH vs installed)
- Refresh pulls mock infrastructure into existing projects
- 15 tests passing: 7 unit (MockHal) + 8 system (SimHal)

templates/basic/test/run_tests.bat

@@ -1,11 +1,37 @@
 @echo off
-setlocal
+setlocal enabledelayedexpansion
 
 set "SCRIPT_DIR=%~dp0"
 :: %~dp0 always ends with \ which breaks cmake quoting ("path\" escapes the quote)
 set "SCRIPT_DIR=%SCRIPT_DIR:~0,-1%"
 set "BUILD_DIR=%SCRIPT_DIR%\build"
 
+:: -- Compiler detection (with vswhere fallback for MSVC) --------------------
+
+set "HAS_COMPILER=0"
+where g++ >nul 2>&1 && set "HAS_COMPILER=1"
+where clang++ >nul 2>&1 && set "HAS_COMPILER=1"
+where cl >nul 2>&1 && set "HAS_COMPILER=1"
+if "%HAS_COMPILER%"=="1" goto :compiler_ok
+
+set "VSWHERE=%ProgramFiles(x86)%\Microsoft Visual Studio\Installer\vswhere.exe"
+if not exist "%VSWHERE%" goto :compiler_ok
+
+for /f "usebackq tokens=*" %%i in (`"%VSWHERE%" -latest -property installationPath`) do (
+    set "VS_PATH=%%i"
+)
+if not defined VS_PATH goto :compiler_ok
+
+set "VCVARS=!VS_PATH!\VC\Auxiliary\Build\vcvarsall.bat"
+if not exist "!VCVARS!" goto :compiler_ok
+
+echo Setting up MSVC environment...
+call "!VCVARS!" x64 >nul 2>&1
+
+:compiler_ok
+
+:: -- Build and test ---------------------------------------------------------
+
 if "%1"=="--clean" (
     if exist "%BUILD_DIR%" (
         echo Cleaning build directory...
@@ -18,7 +44,7 @@ if not exist "%BUILD_DIR%\CMakeCache.txt" (
     cmake -S "%SCRIPT_DIR%" -B "%BUILD_DIR%" -DCMAKE_BUILD_TYPE=Debug
     if errorlevel 1 (
         echo FAIL: cmake configure failed.
-        echo   cmake is required for host-side tests.
+        echo   cmake and a C++ compiler are required for host-side tests.
         echo   Run 'anvil doctor' to see install instructions.
         exit /b 1
     )