display_gif_embedded: 15.4 -> 20.6 fps

- FPS/refresh/decode counter via LVGL monitor_cb (printed once per second)
- LVGL logging off, per-loop memory prints removed, loop delay 5 -> 1 ms
- double draw buffer (render while previous chunk is flushed)
- compile with -O2 instead of -Os

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

CLAUDE.md

@@ -96,6 +96,11 @@ Cheap dual-mode clones typically expose BLE service `FFF0` (notify `FFF1`, write
 ## Gotchas / observations
 - **LVGL GIF bug (patched locally):** LVGL 8.3.11 `gifdec.c` (`gd_open_gif`) filled the canvas *opaque* with the GIF's background color. GIFs with a transparent background (e.g. `loligod.gif`, bg = RGB 28,29,49) showed that dark gray except where "restore to background" frames had cleared to transparent (→ screen black). **Patched** in the vendor copy `2-Program Example/Demo_Arduino/library/lvgl/src/extra/libs/gif/gifdec.c`: initial canvas alpha 0x00 (marked `LOCAL PATCH`). Re-apply if LVGL is ever replaced/upgraded.
 - Most GIFs in `assets/gifs` are optimized (transparent unchanged pixels, disposal 1/2); `loligod.gif` has a fully transparent background (~84 % of each frame).
+- **GIF performance** (360×360 loligod, embedded, measured with a `monitor_cb` FPS counter in `display_gif_embedded`):
+  baseline 15.4 fps (decode 27 ms + refresh 31 ms) → no LVGL log / no per-loop prints: no change → `vTaskDelay(1)`: 16.5 → **double draw buffer** (2×72 rows internal RAM): 19.5 (refresh 21 ms) → **`-O2`** instead of `-Os`: **20.6 fps** (refresh 19 ms, decode ~24 ms). GIF's own rate is 23.7 fps. From SD card it was only ~1.4 fps.
+  Remaining limit is LVGL's single-threaded GIF decode; going further would need decoding on the other core or a lighter format (e.g. pre-converted frames/MJPEG).
+- **SD card (user's 487 MB SDSC)** intermittently fails to init (`sdmmc_init_ocr ... 0x107`) after resets; once stuck only a power cycle helps (card VDD is hard-wired to 3V3). Recommend a newer SDHC card.
+- Reading boot logs: USB-CDC — reset via DTR=0/RTS pulse, then reopen the port (it re-enumerates).
 - LVGL version: all JC3636W518 projects use the vendor-bundled **LVGL 8.3.11**. LVGL 9 would require porting `scr_st77916.h` (display/touch driver glue).
 - Display is round: corners of the 360×360 framebuffer are not visible — design UI within a 360 px circle.
 - Compiling a 13.9 MB GIF into the firmware will not fit "Huge APP" (3 MB); large media should live on SD (or in a custom partition).