Rewrite Nano serial parser as line based, fix protocol bugs

- Commands run on '\n' instead of busy-waiting for a byte count, which
  fixes s/F/T waiting for the next command and P drawing 0xFF garbage
- Invalid bitmaps no longer leak their data as commands; bitmap data is
  streamed so full-screen bitmaps still fit
- Store flip time as uint16 with magic marker, validate 50-2000 us
- Overflow-safe millis() handling for the periodic refresh
- Run the full refresh only after D or when serial is idle, so incoming
  data is not lost while the redraw blocks
- Put the boot refresh measurement behind MEASURE_REFRESH
- Fix DEBUG hex output and use F() strings to save RAM
- Driver: direct mode now updates the old buffer instead of re-setting
  the new one

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

lib/BROSE9323/BROSE9323.cpp

@@ -151,22 +151,11 @@ void BROSE9323::drawPixel(int16_t x, int16_t y, uint16_t color)
 
     _strobe();
 #if !defined(__AVR_ATmega168P__) && !defined(__AVR_ATmega168PB__) && !defined(__AVR_ATmega168__)
+    // The dot is flipped now, so the next display() does not have to flip it again
     if (color)
-    {
-      if (_new_buffer[y * _buffer_width + x / 8] & (1 << (x & 7)))
-      {
-        return;
-      }
-      _new_buffer[y * _buffer_width + x / 8] |= 1 << (x & 7);
-    }
+      _old_buffer[y * _buffer_width + x / 8] |= 1 << (x & 7);
     else
-    {
-      if (~_new_buffer[y * _buffer_width + x / 8] & (1 << (x & 7)))
-      {
-        return;
-      }
-      _new_buffer[y * _buffer_width + x / 8] &= ~(1 << (x & 7));
-    }
+      _old_buffer[y * _buffer_width + x / 8] &= ~(1 << (x & 7));
 #endif
   }
 #endif