Preserve edge topology when exporting PLY meshes

[humanbeingZ]

Summary

• advertise edge-index support for PLY export
• preserve edge connectivity through the save-attributes dialog
• keep edge topology implicit because the dialog only exposes optional attributes

Problem

When a PLY containing explicit edges is saved from MeshLab, the output contains the remaining vertices but omits the edge element. The PLY writer already supports edge serialization and compact endpoint remapping, but IOM_EDGEINDEX is absent from the PLY export capability mask. The save dialog also reconstructs the mask only from visible attribute checkboxes, so topology bits without a checkbox are discarded.

Validation

• built the meshlab and io_base targets successfully
• loaded the built io_base plugin and verified that PLY capability and default masks contain IOM_EDGEINDEX
• round-tripped a synthetic line mesh after deleting a vertex and its incident edges; the reloaded PLY contained 2 vertices, 1 edge, and valid compacted endpoint indices
• manually verified that edited line PLY files retain their remaining lines after saving in MeshLab