[bdbarnett]

Downloading a file from a board was 4 to 6 times slower than uploading it. Each 256-byte chunk of a download costs one round trip with a printed reply, and on CircuitPython a reply costs far more than a silent write, so 80 round trips for 20 KB dominated. Chunk size was the lever; the reply was also up to 4x the data as repr of binary.

This reads in 2048-byte chunks and has the board send each chunk as base64 (binascii), checked with a strict character pattern and the existing length check. A board without binascii keeps the old 256-byte repr path, and a board that raises MemoryError gets the chunk halved down to 256.

Measured on an ESP32-S3 running CircuitPython 11 over USB serial (COM17), same board and transport, mpftp put/cp --no-verify, whole command including about 5 s of start-up, files compared with cmp afterwards:

size	put	get before	get after
4 KB	6.9 s	17.1 s	8.7 s
20 KB	15.1 s	64.5 s	17.4 s
100 KB	53.5 s	309.5 s	55.1 s

A 20 KB cp with the default verify, which reads the file a second time on this board, went from 130 s to 67 s.

Tests: python -m unittest cli.tests.test_read_file_checked (new cases for chunk count, no-binascii fallback, small heap and base64 noise).

Closes #133.