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>

src/nano/command_parser.cpp

@@ -71,7 +71,6 @@ void CommandParser::feed(char c)
 		return;
 	}
 	_line[_line_len++] = c;
-	_frame_ended = false;
 
 	// Bitmap header complete: draw the pixel data as it arrives
 	if (_line_len == BITMAP_HEADER_LEN && isBitmapCommand(_line[0]))
@@ -82,11 +81,6 @@ void CommandParser::feed(char c)
 	}
 }
 
-bool CommandParser::frameEnded() const
-{
-	return _frame_ended;
-}
-
 // Run one complete command line (without the '\n')
 void CommandParser::_runLine()
 {
@@ -102,6 +96,10 @@ void CommandParser::_runLine()
 		// Complete bitmap headers are handled in feed()
 		_error(F("Incomplete bitmap header"));
 		break;
+	case 'c':
+	case 'C':
+		_checksum();
+		break;
 	case 'e':
 	case 'E':
 		_effect();
@@ -110,21 +108,18 @@ void CommandParser::_runLine()
 	case 'G':
 		_gap();
 		break;
+	case 'i':
+	case 'I':
+		_refreshInterval();
+		break;
 	case 'n':
 	case 'N':
 		_pulses();
 		break;
 	case 'd':
 	case 'D':
-		// D1: a frame of moving content, shown without the effect. More frames follow
-		// soon, so the periodic full redraw waits until the serial line is idle.
-		if (_line[1] == '1')
-			_display.display(false, true);
-		else
-		{
-			_display.display();
-			_frame_ended = true;
-		}
+		// D1: a frame of moving content, shown without the effect
+		_display.display(false, _line[1] == '1');
 		break;
 	case 'f':
 	case 'F':
@@ -320,3 +315,28 @@ void CommandParser::_effect()
 	_display.setEffect(effect, t, flags);
 	saveEffect(effect, t, flags);
 }
+
+// C: replies "C xxxx", the bufferCrc() of the image received so far as 4 hex digits. The
+// ESP32 sends it after a frame and compares it with what it sent.
+void CommandParser::_checksum()
+{
+	uint16_t crc = _display.crc();
+	char text[7] = "C ";
+	for (uint8_t i = 0; i < 4; i++)
+		text[2 + i] = "0123456789ABCDEF"[(crc >> (12 - 4 * i)) & 0xF];
+	text[6] = '\0';
+	_reply.println(text);
+}
+
+// Immm (mmm = decimal minutes between full redraws, 0 = never, up to 1440), stored in EEPROM
+void CommandParser::_refreshInterval()
+{
+	long m = atol(_line + 1);
+	if (_line_len < 2 || m < 0 || m > REFRESH_MINUTES_MAX)
+	{
+		_error(F("Refresh interval out of range (0-1440 min)"));
+		return;
+	}
+	_refresh_minutes = m;
+	saveRefreshMinutes(m);
+}