Send only the changed dots of a frame to the Nano

Frames were sent as F0 plus one s command per yellow dot (thousands of
characters). During scrolling that overran the Nano's serial buffer, and
half-received frames redrew the whole screen. showBitmap() now sends two B
commands over the changed rectangle (dots off, dots on), or one B command for
a full frame, and the pacing counts the characters sent.

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

lib/BROSE9323/BROSE9323.h

@@ -4,6 +4,9 @@
 #include <Adafruit_GFX.h>
 
 #include "dot_sequence.h"
+#ifdef ESPSERIAL
+#include "frame_commands.h"
+#endif
 
 //#define FLIPDOT_PLCC_ADAPTER
 // ESPSERIAL is set per target via build_flags in platformio.ini (-D ESPSERIAL for the ESP32)
@@ -103,6 +106,11 @@ public:
     BROSE9323(uint8_t, uint8_t, uint8_t, uint16_t ft = 280);
 #ifdef ESPSERIAL
     void begin(Stream * = &Serial);
+    // Shows a full screen bitmap (drawBitmap format) and ends the frame (D, or D1 when
+    // instant). changes_only: only the dots that differ from the last frame are sent
+    // (frame_commands.h); otherwise the whole screen. Returns the characters sent (0:
+    // nothing changed, nothing sent).
+    size_t showBitmap(const uint8_t *bitmap, bool changes_only, bool instant = false);
 #endif
     void begin(void);
     // Shows the buffer: the changed dots, all with force. instant: without the refresh