feat: add HighlightCorners mode to Prop3D and document Prop3D class functionality

docs/code/vtk/vtk_Prop3D.md

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+# Prop3D
+
+`uLib::Vtk::Prop3D` is a bridge class that wraps VTK 3D representations (`vtkProp`, `vtkProp3D`) and integrates them into the `uLib` object model. It allows the framework to manage visual objects, synchronize them with underlying data models, and expose display-specific properties to the GUI.
+
+## Inheritance
+`uLib::Vtk::Prop3D` : `uLib::Object`
+
+## Key Functionalities
+
+### VTK Integration
+The class provides access to the underlying VTK objects:
+- `GetProp()`: Returns the `vtkProp`.
+- `GetProxyProp()`: Returns the `vtkProp3D`.
+- `GetParts()` / `GetProps()`: Returns `vtkPropCollection` for composite objects.
+
+### Model-View Synchronization
+`Prop3D` ensures that the visual representation stays in sync with the domain model:
+- `Update()`: Synchronizes the VTK representation based on current internal state and properties. Should be called when model data changes.
+- `SyncFromVtk()`: Updates internal state using data from the VTK representation (e.g., after user interaction via gizmos in the 3D view).
+- `GetContent()`: Returns the `uLib::Object` that this `Prop3D` represents visually.
+
+### Visual Appearance
+- **Color & Opacity**: `SetColor(r, g, b)` and `SetOpacity(alpha)`.
+- **Selection**: `SetSelectable(bool)` and `SetSelected(bool)` to manage interactivity and highlighting.
+- **BBox/Scale**: `ShowBoundingBox(bool)` and `ShowScaleMeasures(bool)`.
+
+### Rendering Modes
+The rendering style can be controlled via the `Representation` enum:
+- `Points`
+- `Wireframe`
+- `Surface`
+- `SurfaceWithEdges`
+- `Volume`
+- `Outline`
+- `Slice`
+
+### Display Properties System
+`Prop3D` implements a system to expose specific properties (often marked as `hrp` - human readable properties) to a property editor in the GUI.
+
+- `GetDisplayProperties()`: Returns the list of properties registered for display.
+- `RegisterDisplayProperty(uLib::PropertyBase*)`: Adds a property to the display list.
+- `serialize_display(...)`: A virtual method that subclasses implement to define which properties should be exposed.
+
+#### Activating Display Properties
+To automatically populate the display properties list, the `ULIB_ACTIVATE_DISPLAY_PROPERTIES` macro should be called in the constructor. This triggers `serialize_display` using a `display_properties_archive`.
+
+## Implementation Details
+`Prop3D` uses the Pimpl idiom (via `Prop3DData *pd`) to hide VTK-specific implementation details and reduce header dependencies.