Add Lasso Cut Tool (edit_cut plugin)

[Z3roCo0l]

Summary

A new editing tool that draws a closed polyline to cut through mesh triangles and select the inside region.

Features

• Click to add polyline points on the mesh surface
• Q — cut edges along the polyline and add inside faces to selection
• W — subtract inside faces from selection
• D / A / I — deselect all / select all / invert selection
• Delete — delete selected faces (delegates to MeshLab's standard filter_select with full undo support)
• C — clear polyline, Backspace — undo last point, Escape — clear polyline

The tool appears in its own Dynart Tools toolbar section, separate from the standard Edit toolbar.

Implementation

• New plugin: src/meshlabplugins/edit_cut/
• Uses the edit plugin global parameter system (from PR Add global parameter system for edit plugins, optional CTRL invert selection #1639) for settings
• Face deletion is delegated to filter_select rather than implementing custom mesh modification
• Dynart toolbar added to mainwindow_Init.cpp for organizing custom tools

Test plan

• Load a mesh, activate Lasso Cut tool
• Draw a polyline with clicks, verify visual feedback
• Press Q to cut — verify edges are split along polyline and inside faces are selected
• Press W to subtract — verify selection is reduced
• Press Delete — verify selected faces are removed
• Verify Backspace undoes last point, C clears polyline, Escape clears polyline
• Build on Windows, Linux, macOS

Note: This is PR 2 of 3 (split from #1638). Depends on #1639 (global parameter system). PR 3 (Undo/Redo) builds on this.