Fix #8647: Reload surface mesh with selection in Lab demo

[chinanuj]

Description

Fixes #8647.

This PR addresses the bug where reloading a surface mesh from a file either crashes the Lab demo or fails to reload dependent items (such as selection_item). As noted in the issue description, MainWindow::reloadItem() previously did not account for these dependencies, causing them to be permanently destroyed when the parent mesh was replaced.

Changes Made

Because Scene_polyhedron_selection_item intentionally self-destructs when its parent mesh is removed, a simple C++ pointer dependency check was insufficient. This PR introduces a robust "Cache and Restore" mechanism using Qt's dynamic properties:

• MainWindow.cpp: Implemented caching inside reloadItem(). The function now scans the scene for dependent items using a parent_mesh_name property tag. It caches their source filename and loader_name before the parent is deleted. After the new parent mesh is loaded and highlighted via setSelectedItem(), the dependencies are explicitly restored from the cached file paths.
• Selection_plugin.cpp: Updated the load() function to safely rely on the default constructor for memory mapping (preventing 0-vertex load failures and segfaults). Added a dynamic tag (setProperty("parent_mesh_name", target_item->name())) to ensure the selection can be reliably tracked and restored by the main window across reloads.

Testing Performed

1. Loaded a surface mesh (.off) into the scene and highlighted it.
2. Loaded a generated .selection.txt file (confirmed vertices and colors applied successfully).
3. Right-clicked the parent mesh and selected Reload Item from File.
4. Result: The parent mesh updates successfully without crashing, and the dependent selection items are instantly restored and reattached to the new geometry.

Checklist

• Code compiles successfully locally.
• Tested the reload sequence with selection files.
• Removed all debug print statements.