Fully implement SceneGraph as central scene representation

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SceneGraph is partially defined but needs complete implementation to become
the core data structure for ReUseX spatial reasoning. Required features:

1. Hierarchical object relationships (rooms contain walls, walls contain windows)
2. Spatial queries (find objects in bounding box, nearest neighbors)
3. Attribute management via Registry (materials, labels, metadata)
4. Serialization/deserialization to HDF5 format
5. Integration with CellComplex for 3D reconstruction pipeline
6. Visitor pattern for scene traversal and operations
   Major architectural work but enables modular scene manipulation

Line: 62

ReUseX/libs/reusex/include/ReUseX/geometry/SceneGraph.hpp

Lines 59 to 69 in f6e1667

	namespace ReUseX::geometry {
	// TODO: Fully implement SceneGraph as central scene representation
	// category=Geometry estimate=2w
	// SceneGraph is partially defined but needs complete implementation to become
	// the core data structure for ReUseX spatial reasoning. Required features:
	// 1. Hierarchical object relationships (rooms contain walls, walls contain windows)
	// 2. Spatial queries (find objects in bounding box, nearest neighbors)
	// 3. Attribute management via Registry (materials, labels, metadata)
	// 4. Serialization/deserialization to HDF5 format