Reset Spoolman link when filament metadata changes on loaded slot

Track a fingerprint (material|name|vendor|color) per slot. If the
metadata changes while a spool is linked — without an RFID state
transition — auto-unlink Spoolman. Catches manual spool swaps that
don't trigger a state=0 transition. Also extends unlink detection to
cover any loaded→empty transition (not just RFID→non-RFID).

Cherry-picked from davidkinnes/CFSync:
- acae56e Reset Spoolman link when filament metadata changes on loaded slot