Lifted from tools/screenshot.py so the web UI cannot drift from the CLI
screenshotter. Verified unpack() is identical to the original across 200
randomised trials, and the PNG IHDR matches byte for byte.
encode_png() returns bytes instead of writing a file, and drops to zlib level 6:
at streaming rates the CPU saving beats the last few bytes on loopback.
mckero
1906e110f1