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>

CLAUDE.md

@@ -147,7 +147,10 @@ See README.md for the overview.
   old hardcoded clock (also the default and the HA "Clock" option).
 - Moving pages: `renderPage()` returns when the page changes next (scroll step, second);
   `FlipdotFrames` sends those frames as `D1` (no effect) and paces them by the estimated
-  flip time, because the Nano has no acknowledgement. Character 127 is the degree sign in
+  receive and flip time, because the Nano has no acknowledgement. Full screen frames go
+  through `BROSE9323::showBitmap()`: only the changed dots, as two `B` commands over the
+  changed rectangle (`lib/BROSE9323/frame_commands.h`, tested with the Nano parser in
+  `test_nano`). Don't send frames pixel by pixel (`s` commands are 7 characters per dot). Character 127 is the degree sign in
   page texts and fonts (UTF-8 ° in JSON).
 - Fonts use the "FDF1" format of `logic/font.h`; the web page writes the same format.
   `logic/builtin_fonts.cpp` is generated: change `tools/make_builtin_fonts.py` and rerun it.