Persistent subprocess model: gwbasic reads BASIC from stdin and writes
output to stdout in piped mode (no banner, no prompts, unbuffered).
Sentinel protocol (PRINT "<<<GWDONE>>>") delimits output per cell.
Features: state persistence across cells, error detection and reporting,
tab completion for GW-BASIC keywords, %reset/%timeout/%new magic
commands, Ctrl+C forwarding, automatic process restart.
Includes setup.py for pip install, kernel.json spec, install script,
and 10-test smoke suite (all passing).
