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/CMakeLists.txt

@@ -209,6 +209,11 @@ if(BUILD_TESTING AND CMAKE_SYSTEM_NAME STREQUAL CMAKE_HOST_SYSTEM_NAME)
     target_link_libraries(port_spherelib_test PRIVATE port_platform)
     add_test(NAME port.spherelib COMMAND $<TARGET_FILE:port_spherelib_test>)
 
+    # grp.SaveScreenShot: the host frame request and the 40250 JPEG write behind it.
+    add_executable(port_screenshot_test ${PROJECT_SOURCE_DIR}/tests/port/port_screenshot_test.cpp)
+    target_link_libraries(port_screenshot_test PRIVATE port_platform)
+    add_test(NAME port.screenshot COMMAND $<TARGET_FILE:port_screenshot_test>)
+
     # 批次 2V1: Packet.h struct sizes against the classic wire layouts.
     add_executable(port_packet_test ${PROJECT_SOURCE_DIR}/tests/port/port_packet_test.cpp
         ${PROJECT_SOURCE_DIR}/tests/port/port_packet_test_wire.cpp)

src/port/EterLib/JpegFile.h

@@ -0,0 +1,9 @@
+#pragma once
+
+#include <stdio.h>
+
+int jpeg_save(unsigned char*data_d, int width, int height, int quality, const char*filename);
+int jpeg_save_to_file(unsigned char*data_d, int width, int height, int quality, FILE*fi);
+int jpeg_save_to_mem(unsigned char*data_d, int width, int height, int quality, unsigned char*dest, int destsize);
+int jpeg_load(const char*filename, unsigned char**dest, int*width, int*height);
+int jpeg_load_from_mem(unsigned char*_data, int size_d, unsigned char*dest, int width, int height);

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)

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;
 }