Improve unexpected vertex snapping when editing meshes

When moving vertices in Edit Mesh mode, the vertices may sometimes snap unexpectedly or become difficult to move, even when the intended movement should seemingly be possible.

This behavior can be reproduced with Spineboy’s `head` mesh.

Video: https://github.com/user-attachments/assets/0b6c010e-a163-433e-af5c-cd25cd525ca1

The video above was recorded using Spine 4.3.23.

Defined mesh edges, shown in cyan or orange, are not allowed to cross. This constraint is necessary for Spine to maintain valid mesh triangulation. However, from the user’s perspective, it is unclear why the particular movement shown in the video causes such a large snap.

It would be helpful if the mesh hull could be edited without encountering unexpected snapping like this.

**Related forum thread:**
https://esotericsoftware.com/forum/d/30499-%E6%97%A0%E6%B3%95%E7%A7%BB%E5%8A%A8%E7%BD%91%E6%A0%BC

Reproduce the issue in Spine 4.3.23 by opening Spineboy’s head mesh in Edit Mesh mode and following the movement shown in the linked video. Investigate how mesh hull editing handles defined edges that cannot cross; done means vertices can be moved without an unexpected large snap while valid mesh triangulation is preserved.

Written by the indexing model from the issue text.