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/shim/win32/shlobj.h

@@ -2,11 +2,33 @@
 // Win32 shell-folder API used only by the screenshot command in grp.
 #include <windows.h>
 
+#include <cstdio>
+#include <cstdlib>
+#include <sys/stat.h>
+#include <unistd.h>
+
 #define CSIDL_PERSONAL 0x0005
 
+// CSIDL_PERSONAL ("My Documents"): $HOME/Documents where it exists (desktop), otherwise the working
+// directory, which is the writable Client directory on Android. grp.SaveScreenShot appends
+// "\\METIN2\\"; CreateDirectory and jpeg_save take '\\' as a separator like Win32 does.
 inline BOOL SHGetSpecialFolderPath(void*, char* path, int, BOOL)
 {
-	if (path)
+	if (!path)
+		return FALSE;
+	path[0] = '\0';
+	struct stat st;
+	const char* home = std::getenv("HOME");
+	if (home && home[0])
+	{
+		std::snprintf(path, MAX_PATH, "%s/Documents", home);
+		if (::stat(path, &st) == 0 && S_ISDIR(st.st_mode))
+			return TRUE;
+	}
+	if (!::getcwd(path, MAX_PATH))
+	{
 		path[0] = '\0';
-	return FALSE;
+		return FALSE;
+	}
+	return TRUE;
 }