Add object picking via OptiX single-ray raycasts

[brendancol]

Summary

• Add RTX.pick(origin, direction) method that fires a single OptiX ray and returns hit info (geometry ID, position, normal, primitive/instance IDs)
• Add _screen_to_ray() to InteractiveViewer for window-to-world ray conversion
• Right-click in the interactive viewer now picks geometry and prints hit info to the terminal
• Add ViewerProxy.pick(screen_x, screen_y) for programmatic picking from the REPL (v.pick(400, 300))
• Add help text entry for right-click picking

Test plan

• Run an explore session with placed geometry (e.g., buildings) and right-click on objects to verify geometry ID, position, and primitive info are printed
• Right-click on terrain to confirm it reports 'terrain' as geometry ID
• Right-click on sky (miss) to confirm "no geometry hit" message
• Test REPL access via v.pick(400, 300) and verify returned dict
• Run existing tests: pytest rtxpy/tests/test_simple.py -k instance_ids