port(2A): port/common Win32 layer, port_logic target and header gate

- Win32Types.h: fixed-width Win32 scalars, pointer-sized handles, static_asserts
- Win32Crt.{h,cpp}: MSVC _snprintf truncation contract, stricmp, timeGetTime wrap
- port_logic static lib linked into libmtgodot; standalone MTGODOT_BUILD_PORT build
- port_header_gate: every port/**.h compiles alone (twice) with its lib's
  40250 Distribute|Win32 defines (NDEBUG, USE_LOD, _DISTRIBUTE)
- script/port_gate.sh: macOS/Android/iOS/Windows(mingw) PASS, Linux BLOCKED

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

extension/src/port/common/Win32Crt.cpp

@@ -0,0 +1,63 @@
+#include "Win32Crt.h"
+
+#if !defined(_WIN32)
+#include <cctype>
+#include <chrono>
+#include <cstdio>
+#include <vector>
+
+char* _strlwr(char* s)
+{
+	for (char* p = s; *p; ++p)
+		*p = static_cast<char>(std::tolower(static_cast<unsigned char>(*p)));
+	return s;
+}
+
+char* _strupr(char* s)
+{
+	for (char* p = s; *p; ++p)
+		*p = static_cast<char>(std::toupper(static_cast<unsigned char>(*p)));
+	return s;
+}
+
+int _vsnprintf(char* buf, size_t count, const char* fmt, va_list ap)
+{
+	va_list probe;
+	va_copy(probe, ap);
+	const int len = std::vsnprintf(nullptr, 0, fmt, probe);
+	va_end(probe);
+	if (len < 0)
+		return -1;
+
+	const size_t need = static_cast<size_t>(len);
+	if (need < count)
+		return std::vsnprintf(buf, count, fmt, ap);
+
+	std::vector<char> full(need + 1);
+	std::vsnprintf(full.data(), full.size(), fmt, ap);
+	std::memcpy(buf, full.data(), count);
+	return need == count ? len : -1;
+}
+
+int _snprintf(char* buf, size_t count, const char* fmt, ...)
+{
+	va_list ap;
+	va_start(ap, fmt);
+	const int ret = _vsnprintf(buf, count, fmt, ap);
+	va_end(ap);
+	return ret;
+}
+
+DWORD timeGetTime()
+{
+	using namespace std::chrono;
+	const auto ms = duration_cast<milliseconds>(steady_clock::now().time_since_epoch()).count();
+	return static_cast<DWORD>(static_cast<uint64_t>(ms));
+}
+
+DWORD GetTickCount()
+{
+	return timeGetTime();
+}
+
+#endif  // !_WIN32