Klipper shutdown: BOX_TEMP_SET VT4 references non-existent box_stepper slot4 when printing from external spool

[azbinden-dev]

Firmware: Plus4 v1.7.1 (EU 230V) · Slicer: QIDI Studio 02.07.02.60 · Hardware: 1× QIDI Box (box_count = 1)

Summary

Printing from the external spool in a project with 5 filaments causes an
immediate Klipper shutdown at print start.

Reproduce

1. One QIDI Box attached; sync filaments from printer. Studio populates
   4 box slots + external spool as filament 5.
2. Assign a model to filament 5 (external spool). Load that filament externally.
3. Slice and print.

Result

Studio emits, immediately before the machine start G-code:

DISABLE_BOX_HEATER
BOX_TEMP_SET VT4=0

box_extras resolves VT4 as a physical slot and Klipper shuts down:

Internal error on command:"BOX_TEMP_SET"
File "box_extras.py", line 591, in extras.box_extras.BoxExtras.set_box_temp
File "/home/mks/klipper/klippy/klippy.py", line 99, in lookup_object
    raise self.config_error("Unknown config object '%s'" % (name,))
configparser.Error: Unknown config object 'box_stepper slot4'

Cause

VT is the project filament index. With 5 filaments the external spool is
index 4, but a single Box only configures box_stepper slot0–slot3. The
firmware already identifies the external spool as slot 16 elsewhere —
saved_variables.cfg contains filament_slot16, color_slot16, vendor_slot16.

Two Boxes masks the crash: slot4 exists, so the dryer target is silently
applied to the wrong slot.

Requested fix

1. QIDI Studio: emit the external spool as VT16, matching firmware numbering.
2. box_extras.set_box_temp: bounds-check the slot index and warn on an
   unknown slot rather than calling invoke_shutdown.

Workaround

Slice the external-spool part in its own single-filament project so the
index is 0.