AVR probe returns empty / termios fails: check /dev/ttyUSB0 is a char device before assuming driver or AVR fault

Trap: AVR probe returning empty responses with AVR physically ON looks like a driver bug (pl2303 not recognizing PL2303GL) or AVR comms failure. But if /dev/ttyUSB0 is a regular file (created by shell <> when char node was absent), all writes go to disk silently, reads return stale TX echo bytes, and stty returns 'Inappropriate ioctl' — identical symptoms. Fix: always check [ -c /dev/ttyUSB0 ] first. The is_on() method in chromecast_avr_bridge.py uses VOL UP (side-effect-free after OSD timeout) as probe; checks for b'AVRACK' in response; returns True/False/None. Drains stale buffer before sending. Exit code 3 from SSH = not a char device → is_on() returns None.