Add 12-OBD-Speed: ELM327 speed over BLE on a GIF background

- speed page: 120 px antialiased number (lv_font_conv) + 0-200 km/h arc
  gauge over ezgif-5-cb56986bc7.gif (embedded via board_build.embed_files)
- swipe-left menu: scan, OBD-like adapters first, tap to select; adapter
  remembered in NVS, auto-connect at boot and reconnect every 3 s
- ELM327 init ATZ/ATE0/ATL0/ATS0/ATH0/ATSP0/0100, then 010D polling
- shared ini: -DLV_LVGL_H_INCLUDE_SIMPLE for generated fonts

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

12-OBD-Speed/README.md

@@ -0,0 +1,29 @@
+# OBD Speed (JC3636W518)
+
+Vehicle speed from an ELM327 over BLE, shown as a big number + arc gauge on top of an animated GIF.
+Uses the shared board library and LVGL 9.6 (`../shared`), NimBLE-Arduino 2.5.1.
+
+## Use
+- **Speed page** — speed in km/h (`--` = no data: not connected, or ignition off), arc gauge 0–200 km/h,
+  status at the bottom (adapter name when running, otherwise what it is doing).
+- **Swipe left → adapter menu** — scans while open. OBD-like adapters (name OBD/ELM/…, or service FFF0/FFE0/18F0)
+  are listed first in green. **Tap one** to select it: it is remembered (NVS) and connected immediately.
+  **Forget** clears the remembered adapter, **Done** (or swipe right) returns to the speed page.
+- On power-up the remembered adapter is connected automatically; if the link drops or the adapter is not
+  there (car off), it retries every 3 s. With no adapter remembered, the device starts on the menu page.
+
+## How it talks to the ELM327
+Service `FFF0`, notifications + commands on `FFF1` (verified with the user's "OBDII" adapter, see
+`../11-BLE-Scanner/results/2026-10-23_car_test.txt`); other adapters: first notify+write pair.
+Init: `ATZ`, `ATE0`, `ATL0`, `ATS0`, `ATH0`, `ATSP0`, `0100`; then `010D` (speed) is polled continuously.
+Serial (115200) prints status changes as `[obd] ...`.
+
+## Files
+| File | What |
+|---|---|
+| `src/main.cpp` | LVGL UI (tileview: speed page + menu page) |
+| `src/obd_link.cpp/.h` | BLE scan, connect/reconnect, ELM327 init and speed polling (background task on core 0) |
+| `src/font_speed_120.c` | Montserrat Medium 120 px, digits and `-`, 4 bpp (generated with `lv_font_conv`, see header) |
+| `data/bg.gif` | Background (`ezgif-5-cb56986bc7.gif`, 360×360), linked into the firmware via `board_build.embed_files` |
+
+Replace `data/bg.gif` with any 360×360 GIF to change the background (keep it under ~3 MB).