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>

src/esp32/display/frames.h

@@ -12,9 +12,9 @@ class FlipdotFrames : public Frames
 public:
 	FlipdotFrames(BROSE9323 &display, PageResources &resources);
 
-	// The page set with setDynamicPage(), drawn for this time. Not sent when it looks the
-	// same as the last frame (unless redraw). Moving pages are sent without the refresh
-	// effect, and not faster than the Nano can flip the dots.
+	// The page set with setDynamicPage(), drawn for this time. Only the changed dots are
+	// sent (nothing when it looks the same), unless redraw. Moving pages are sent without
+	// the refresh effect, and not faster than the Nano can receive and flip them.
 	uint32_t dynamicPage(const LocalTime &time, uint32_t now, bool redraw) override;
 	// "No WiFi" with the time below it
 	void noWifi(int hour, int minute) override;
@@ -37,12 +37,11 @@ private:
 	BROSE9323 &_display;
 	PageResources &_resources;
 	DynamicPage _dynamic;
-	uint8_t _last_dynamic[BITMAP_BYTES] = {};
 	uint32_t _dot_us = 1000; // per changed dot: pulses x (flip time + gap)
 	uint8_t _page[BITMAP_BYTES] = {};
 	uint8_t _live[BITMAP_BYTES] = {};
 	uint8_t _off[BITMAP_BYTES] = {}; // all zero: blank
 
-	void _drawFullScreen(const uint8_t *bitmap, bool instant = false);
-	uint32_t _flipMs(const uint8_t *bitmap, bool all) const;
+	void _drawFullScreen(const uint8_t *bitmap);
+	uint32_t _show(const uint8_t *bitmap, bool changes_only, bool instant);
 };