When a printer shuts down it sends a final MQTT message with
tray_exist_bits=0 and power_on_flag=false. The tray_exist_bits
clearing code processed this all-zero value, wiping every AMS
slot's filament data. On reconnect, the auto-unlink check saw
empty tray data (no color, no type) and deleted all spool
assignments as "fingerprint mismatch".
Fix: skip tray_exist_bits slot clearing when power_on_flag is
false. Defaults to true when absent for backwards compatibility.
Adds 3 regression tests covering shutdown preservation, genuine
removal still working, and missing power_on_flag fallback.
maziggy
488f66310f