Printing via OrcaSlicer "Upload and print" failed or fed the wrong spool
when the slot below the used filament was empty (e.g. Filament 4 with slot
3 empty); all-slots-full worked.
_start_print -> _build_auto_ams_box_mapping keeps the AMS mapping positional
(entry N = TN) by inserting a placeholder at each gap. The placeholder's
ams_index pointed at the gap's own index, which for an empty slot references
a physically empty tray. The printer rejects a mapping entry aimed at an
empty tray even for a tool the GCode never calls, so the print broke.
Point gap placeholders (ams_index/color/material) at a definitely-loaded
fallback tray (the highest loaded slot) instead. Positional alignment is
preserved; the all-full path is unchanged.
Adds tests/test_auto_ams_box_mapping_empty_slot.py (invariant: every mapping
entry references a loaded/status-5 tray).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
