Fix dense images on the Nano and the off image in slow mode

- Nano: B data is drawn fast enough for the serial line. A hex digit arrives
  every 87 us; with a division and four drawPixel() calls per digit a lit
  digit took about 105 us, so dense frames (all yellow, the panda bitmap)
  overflowed the 64 byte receive buffer: frames not shown or wrong, "Line
  too long", frame check repairs that failed the same way. Now the parser
  counts columns and rows and BROSE9323::drawDots() sets the 4 dots
  (about 20 us).
- ESP32: Power off and the start of the night draw the off image also when
  slow mode had the power off already (it powers on for it, once)
- Tests: drawDots() against drawPixel(), off image in slow mode
- Version 2.19.3

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

lib/BROSE9323/BROSE9323.cpp

@@ -290,6 +290,25 @@ void BROSE9323::printBuffer(void)
   }
 }
 
+void BROSE9323::drawDots(int16_t x, int16_t y, uint8_t bits, bool color)
+{
+  if (!(bits & 0xF) || y < 0 || y >= height())
+    return;
+  // Never used in direct mode (B commands), so only the buffer changes
+  _drawing = true;
+  uint8_t *row = _new_buffer + y * _buffer_width;
+  for (uint8_t mask = 8; mask; mask >>= 1, x++)
+  {
+    if (!(bits & mask) || x < 0 || x >= width())
+      continue;
+    uint8_t bit = 1 << (x & 7);
+    if (color)
+      row[x >> 3] |= bit;
+    else
+      row[x >> 3] &= ~bit;
+  }
+}
+
 void BROSE9323::_selectColumn(uint8_t col)
 {
   if (_active_col == col)