Merging anchors in path tool

It is possible to join anchors with a new segment Ctrl+J. However it is no yet possible to simply merge the anchors (no new segments).

This should work across different layers. This should reuse the merging logic from Ctrl+J:

https://github.com/GraphiteEditor/Graphite/blob/97f8113fe43d41b15ca3a56c300c48b7c8fda5c4/editor/src/messages/tool/common_functionality/shape_editor.rs#L535-L536

This functionality is also available in inkscape and figma.

Start in editor/src/messages/tool/common_functionality/shape_editor.rs at the linked Ctrl+J merging logic. Trace how that operation handles anchors and layers, then determine the path-tool entry point for merging without creating segments. Done means selected anchors can merge across different layers while preserving the existing Ctrl+J behavior.

Written by the indexing model from the issue text.