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.h

@@ -154,6 +154,10 @@ public:
     bool refreshing(void) const { return _refreshing; }
 #endif
 #ifndef ESPSERIAL
+    // Draws the set bits of a hex digit of B data into the buffer (bit 3 at x, bit 0 at
+    // x + 3), like drawPixel() but fast enough for the serial port: dense B lines take
+    // about 23 us per dot with drawPixel(), and a digit arrives every 87 us
+    void drawDots(int16_t x, int16_t y, uint8_t bits, bool color);
     void printBuffer(void);
 #endif
     size_t print(const char *ifsh);