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/mocks/SPI.h

@@ -0,0 +1,55 @@
+/*
+ * SPI.h -- shim for host-side test builds.
+ *
+ * Intercepts #include <SPI.h> so that code referencing SPI compiles
+ * on x86_64. Provides a minimal mock of the Arduino SPI library.
+ *
+ * Generated by Anvil.
+ */
+#ifndef SPI_H_MOCK_SHIM
+#define SPI_H_MOCK_SHIM
+
+#include "mock_arduino.h"
+
+#define SPI_MODE0 0x00
+#define SPI_MODE1 0x04
+#define SPI_MODE2 0x08
+#define SPI_MODE3 0x0C
+
+#define SPI_CLOCK_DIV2   0x04
+#define SPI_CLOCK_DIV4   0x00
+#define SPI_CLOCK_DIV8   0x05
+#define SPI_CLOCK_DIV16  0x01
+#define SPI_CLOCK_DIV32  0x06
+#define SPI_CLOCK_DIV64  0x02
+#define SPI_CLOCK_DIV128 0x03
+
+#define MSBFIRST 1
+#define LSBFIRST 0
+
+struct SPISettings {
+    uint32_t clock;
+    uint8_t bitOrder;
+    uint8_t dataMode;
+    SPISettings() : clock(4000000), bitOrder(MSBFIRST), dataMode(SPI_MODE0) {}
+    SPISettings(uint32_t c, uint8_t o, uint8_t m)
+        : clock(c), bitOrder(o), dataMode(m) {}
+};
+
+class MockSPI {
+public:
+    void begin() {}
+    void end() {}
+    void beginTransaction(SPISettings) {}
+    void endTransaction() {}
+    uint8_t transfer(uint8_t data) { (void)data; return 0; }
+    uint16_t transfer16(uint16_t data) { (void)data; return 0; }
+    void transfer(void* buf, size_t count) { (void)buf; (void)count; }
+    void setBitOrder(uint8_t) {}
+    void setClockDivider(uint8_t) {}
+    void setDataMode(uint8_t) {}
+};
+
+extern MockSPI SPI;
+
+#endif // SPI_H_MOCK_SHIM