display_gif_sd: load GIF from SD into PSRAM once, play from memory (9.8 -> 15.4 fps)

Also enable a 16 KB LVGL stdio read cache (no measurable effect for the GIF).

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

shared/include/lv_conf.h

@@ -1967,7 +1967,7 @@
 #define LV_FS_STDIO_PATH "/sdcard/"
 
 /** Read cache size in bytes for stdio (0 = disabled) */
-#define LV_FS_STDIO_CACHE_SIZE 0
+#define LV_FS_STDIO_CACHE_SIZE 16384  /* JC3636W518: fewer small SD reads (GIF decoding) */
 
 #endif /*LV_USE_FS_STDIO*/