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>

CLAUDE.md

@@ -31,13 +31,21 @@ See README.md for the overview.
   Add new secrets to `secrets.example.h` as placeholders.
 - Hardware is not available here. Builds can be checked, but runtime behaviour can't.
 
-## Known quirks in the serial protocol (Nano `src/nano/main.cpp`)
+## Nano serial parser (`src/nano/main.cpp`)
 
-- `T` waits for more than 6 bytes before parsing, but `T225\n` has only 4 after the `T`.
-  It completes only when the next command arrives.
-- The `T` value is stored with `EEPROM.update` in one byte, so values above 255 µs are
-  truncated after a reboot.
-- `P` (print text) busy-waits for a `\n` and has no timeout.
+- Line based and non-blocking: bytes are collected into a 64-character buffer and a
+  command runs when `\n` arrives. Bitmap (`B`) pixel data is the exception. It is drawn
+  while it arrives, so it can be longer than the buffer. Keep `loop()` free of busy-waits.
+- A full redraw (`display(true)`) blocks for about 0.5 s, which overflows the 64-byte
+  serial receive buffer. Only run it when no data is expected: right after a `D`, or after
+  2 s with no serial data. The ESP sends one frame per second, ending with `D`.
+- Flip time is stored in EEPROM as a magic word at address 0 and a `uint16_t` at
+  address 2. Values outside 50–2000 µs are rejected.
+- The Nano doesn't send an acknowledgement. Whether the Nano's TX is wired to the ESP's RX
+  is unknown, so don't make the ESP wait for replies.
+- To test the parser without hardware, compile `main.cpp` with g++ (Strawberry Perl
+  provides it at `C:\Strawberry\c\bin`) and mock headers for `Arduino.h`, `EEPROM.h` and
+  `BROSE9323.h`.
 
 ## Git