AMS-HT trays (IDs 128-135) are single-slot units, but the snow_slot
computation used `snow_tray % 4 if snow_tray < 128 else -1`, yielding
-1 for AMS HT. This caused a misleading "slot mismatch" debug log on
every MQTT update despite producing the correct tray_now value. Now
returns slot 0 for AMS-HT IDs (128-135), matching the firmware's
tray_now=0 for these single-slot units.