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)

src/commands/doctor.rs

@@ -13,6 +13,7 @@ pub struct SystemHealth {
     pub dialout_ok: bool,
     pub cmake_ok: bool,
     pub cpp_compiler_ok: bool,
+    pub cpp_on_path: bool,
     pub git_ok: bool,
     pub ports_found: usize,
 }
@@ -99,7 +100,14 @@ pub fn check_system_health() -> SystemHealth {
     let cmake_ok = which::which("cmake").is_ok();
 
     // C++ compiler (optional -- for host tests)
-    let cpp_compiler_ok = has_cpp_compiler();
+    let cpp_on_path = which::which("g++").is_ok()
+        || which::which("clang++").is_ok()
+        || which::which("cl").is_ok();
+    let cpp_compiler_ok = if cpp_on_path {
+        true
+    } else {
+        has_cpp_compiler()
+    };
 
     // git
     let git_ok = which::which("git").is_ok();
@@ -115,6 +123,7 @@ pub fn check_system_health() -> SystemHealth {
         dialout_ok,
         cmake_ok,
         cpp_compiler_ok,
+        cpp_on_path,
         git_ok,
         ports_found,
     }
@@ -130,6 +139,20 @@ fn has_cpp_compiler() -> bool {
         if which::which("cl").is_ok() {
             return true;
         }
+        // cl.exe may be installed via VS Build Tools but not on PATH.
+        // Check via vswhere.exe (ships with VS installer).
+        if let Ok(output) = std::process::Command::new(
+            r"C:\Program Files (x86)\Microsoft Visual Studio\Installer\vswhere.exe",
+        )
+        .args(["-latest", "-property", "installationPath"])
+        .output()
+        {
+            let path = String::from_utf8_lossy(&output.stdout);
+            let vcvarsall = format!("{}\\VC\\Auxiliary\\Build\\vcvarsall.bat", path.trim());
+            if std::path::Path::new(&vcvarsall).exists() {
+                return true;
+            }
+        }
     }
     false
 }
@@ -217,8 +240,14 @@ fn print_diagnostics(health: &SystemHealth) {
     }
 
     // C++ compiler
-    if health.cpp_compiler_ok {
+    if health.cpp_on_path {
         println!("  {} C++ compiler", "ok".green());
+    } else if health.cpp_compiler_ok {
+        println!(
+            "  {} C++ compiler  {}",
+            "ok".green(),
+            "(MSVC installed; test.bat will configure automatically)".bright_black()
+        );
     } else {
         println!(
             "  {} C++ compiler  {}",
@@ -655,16 +684,16 @@ fn fix_spec_cmake(pm: Option<&str>) -> FixSpec {
 
 fn fix_spec_cpp(pm: Option<&str>) -> FixSpec {
     match pm {
-        Some("winget") => FixSpec::Auto {
-            prompt: "Install Visual Studio Build Tools via winget?",
-            program: "winget",
-            args: &["install", "--id", "Microsoft.VisualStudio.2022.BuildTools", "-e"],
-        },
         Some("choco") => FixSpec::Auto {
-            prompt: "Install MinGW g++ via Chocolatey?",
+            prompt: "Install MinGW g++ via Chocolatey? (recommended -- lands on PATH)",
             program: "choco",
             args: &["install", "mingw", "-y"],
         },
+        Some("winget") => FixSpec::Auto {
+            prompt: "Install Visual Studio Build Tools via winget? (test.bat will find it automatically)",
+            program: "winget",
+            args: &["install", "--id", "Microsoft.VisualStudio.2022.BuildTools", "-e"],
+        },
         Some("brew") => FixSpec::Manual {
             message: "run: xcode-select --install",
         },
@@ -751,7 +780,7 @@ fn hint_cmake() -> &'static str {
 
 fn hint_cpp_compiler() -> &'static str {
     if cfg!(target_os = "windows") {
-        "install: winget install Microsoft.VisualStudio.2022.BuildTools  (or MinGW g++)"
+        "install: choco install mingw  (or open Developer Command Prompt for MSVC)"
     } else if cfg!(target_os = "macos") {
         "install: xcode-select --install"
     } else {

src/commands/refresh.rs

@@ -16,10 +16,20 @@ const REFRESHABLE_FILES: &[&str] = &[
     "upload.bat",
     "monitor.sh",
     "monitor.bat",
+    "test.sh",
+    "test.bat",
     "_detect_port.ps1",
     "_monitor_filter.ps1",
     "test/run_tests.sh",
     "test/run_tests.bat",
+    "test/CMakeLists.txt",
+    "test/mocks/mock_arduino.h",
+    "test/mocks/mock_arduino.cpp",
+    "test/mocks/Arduino.h",
+    "test/mocks/Wire.h",
+    "test/mocks/SPI.h",
+    "test/mocks/mock_hal.h",
+    "test/mocks/sim_hal.h",
 ];
 
 pub fn run_refresh(project_dir: Option<&str>, force: bool) -> Result<()> {