port: grp.SaveScreenShot via next-frame swapchain readback + JPEG

CPythonGraphic::SaveScreenShot queues a readback of the next presented
frame (ScreenShotHost -> VulkanWindow::request_frame); WriteScreenShot runs
the rest of 40250's function verbatim (X8R8G8B8 rows, SaveJPEG, EXIF tag).
jpeg_save is encoded with stb_image_write. SHGetSpecialFolderPath returns
$HOME/Documents (else cwd); CreateDirectory/_access/jpeg_save accept '\\'.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/port/common/Win32Crt.cpp

@@ -32,9 +32,18 @@ void DeleteCriticalSection(LPCRITICAL_SECTION cs) { cs_mutex(cs).~recursive_mute
 void EnterCriticalSection(LPCRITICAL_SECTION cs) { cs_mutex(cs).lock(); }
 void LeaveCriticalSection(LPCRITICAL_SECTION cs) { cs_mutex(cs).unlock(); }
 
+// Win32 takes '\\' and '/' as separators alike.
+static std::string native_path(const char* path)
+{
+	std::string normalized(path ? path : "");
+	for (char& ch : normalized)
+		if (ch == '\\') ch = '/';
+	return normalized;
+}
+
 BOOL CreateDirectory(LPCSTR path, void*)
 {
-	return mkdir(path, 0777) == 0 ? TRUE : FALSE;
+	return mkdir(native_path(path).c_str(), 0777) == 0 ? TRUE : FALSE;
 }
 
 BOOL DeleteFile(LPCSTR path)
@@ -49,7 +58,7 @@ int _access(const char* path, int mode)
 		m |= W_OK;
 	if (mode & 4)
 		m |= R_OK;
-	return access(path, m);
+	return access(native_path(path).c_str(), m);
 }
 
 int _unlink(const char* path)