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/platform/EterLib/JpegFile.cpp

@@ -0,0 +1,37 @@
+// Platform implementation of EterLib/JpegFile.h. 40250 writes it with libjpeg-9a (JpegFile.cpp: a stdio
+// destination manager, JCS_RGB, jpeg_set_defaults + jpeg_set_quality); here stb_image_write encodes the
+// same input (top-down RGB rows, 3 bytes per pixel) at the same quality into a baseline JFIF file.
+//
+// Only jpeg_save has a caller in 40250 (CPythonGraphic::SaveJPEG, the screenshot key); the other
+// declarations of JpegFile.h stay undefined.
+#include "EterLib/StdAfx.h"
+#include "EterLib/JpegFile.h"
+
+#include <algorithm>
+#include <string>
+
+#define STB_IMAGE_WRITE_IMPLEMENTATION
+#define STB_IMAGE_WRITE_STATIC
+#define STBI_WRITE_NO_STDIO
+#include "../third_party/stb_image_write.h"
+
+namespace
+{
+void write_to_file(void* context, void* data, int size)
+{
+	fwrite(data, 1, size_t(size), static_cast<FILE*>(context));
+}
+}
+
+int jpeg_save(unsigned char*data_d, int width, int height, int quality, const char*filename)
+{
+	// The Win32 CRT takes '\\' as a separator (grp.SaveScreenShot builds "<Documents>\\METIN2\\<time>.jpg").
+	std::string path(filename);
+	std::replace(path.begin(), path.end(), '\\', '/');
+	FILE* fi = fopen(path.c_str(), "wb");
+	if (fi == NULL)
+		return 0;
+	const int ok = stbi_write_jpg_to_func(write_to_file, fi, width, height, 3, data_d, quality);
+	const bool closed = fclose(fi) == 0;
+	return ok && closed ? 1 : 0;
+}