* fix: prevent out-of-bounds crash in Arachne beading interpolation
SkeletalTrapezoidation::interpolate() derives an inset index from `left` but
uses it to index the merged beading, which follows the thicker of left/right.
When the thicker side has fewer insets, the index runs past the end and the
slicer crashes during "Generating walls".
Skip the adjustment when the index is out of range, as the adjacent guards
already do. interpolate() uses no instance state, so make it static and add a
regression test that exercises it directly.
Fixes#14584
dec67345be