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>

test/mocks/BROSE9323.h

@@ -27,6 +27,14 @@ public:
 	void begin() { log("begin"); }
 	void display(bool force = false, bool instant = false) { log("display", force, instant); }
 	void drawPixel(int16_t x, int16_t y, uint16_t color) { log(direct ? "directPixel" : "pixel", x, y, color); }
+	// Recorded as the pixel calls it stands for (the real one is tested in test_driver)
+	void drawDots(int16_t x, int16_t y, uint8_t bits, bool color)
+	{
+		log("dots", x, y, bits);
+		for (int i = 0; i < 4; i++)
+			if (bits & (8 >> i))
+				drawPixel(x + i, y, color);
+	}
 	void fillScreen(uint16_t color) { log("fill", color); }
 	void setDirect(bool d) { direct = d; }
 	void setTiming(uint16_t t) { log("timing", t); }