Fix shifted pictures: non-blocking Nano drawing, frame check, refresh settings

The Nano's periodic full redraw blocked for ~0.8 s. A frame that arrived
meanwhile overflowed its 64-byte serial buffer; lost bytes in a bitmap
shift the rest of the image (picture moved up), and diff frames never
repaired it.

- Nano: all drawing (frames, effects, the dot refresh) runs from update()
  in ~1 ms slices; the refresh pauses while a frame is received or shown
- New commands: C (reply with the image checksum) and I (refresh minutes,
  EEPROM)
- ESP32: NanoMonitor asks for the checksum after frames and sends the whole
  image again on a mismatch, missing answer or Nano restart; counts ERR
  replies. Periodic whole-image resync.
- Settings: "Dot refresh" and "Image resync" intervals (settings v5,
  v4 records are migrated)
- System tab: loop timing, free memory, frame check counts
- test_driver: the real driver against a simulated panel

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

docs/how-it-works.md

@@ -20,7 +20,13 @@
 3. **Home Assistant**: switches, dropdowns and sensors over MQTT, see
    [Home Assistant](home-assistant.md).
 4. **Nano.** The Nano reads commands from the serial line and updates its frame buffer.
-   On `D` it flips only the dots that changed. Every 2 minutes it redraws the whole screen
-   to fix dots that did not flip.
+   On `D` it flips only the dots that changed, in small steps so it never stops reading
+   the serial port. Every 2 minutes (Settings, "Dot refresh") it flips every dot once
+   more to fix dots that did not flip.
+5. **Frame check.** After a frame the ESP asks the Nano for a checksum of its image and
+   compares it with what it sent. When they differ (lost bytes can shift the picture),
+   the Nano doesn't answer, or it restarted, the ESP sends the whole image again. It also
+   does that every 10 minutes (Settings, "Image resync"); only dots that differ flip, so
+   normally nothing changes. The counts are on the System tab.
 
 The serial protocol is described in [src/nano/README.md](../src/nano/README.md).