Estimate memory required for octree construction.

Upper-bound estimate:

• Positions buffer: num_points × 12 bytes (Float32 × 3)
• Reorder map: num_points × 8 bytes (usize)
• Octree nodes: ~100 bytes per estimated node
• Temp buffers: ~50% overhead for intermediate state

Arguments

• num_points — Number of points in the dataset.

Returns

Estimated memory in bytes.