Relation solver places robot

[qianl-nv]

Summary

Let the relation solver place robot embodiments

Detailed description

• Introduce a shared PlacementEntity protocol so the relation solver, object
  placer, and overlap checks operate on both scene objects and embodiments;
  EmbodimentBase and Object now implement it.
• Add USD-derived embodiment placement footprints (embodiment_placement.py)
  and wire Droid so its pose can be solved from spatial relations (e.g. on
  floor, next_to counter) instead of a hardcoded initial pose.
• Expand the relation solver / object placer to write solved poses back to the
  correct Isaac Lab scene entity (robot for embodiments), and expand graph
  conversion to attach embodiment relations.
• Defer the pxr-importing usd_prim_tree import in the pass-2 resolver so
  importing the generation agent no longer pulls in pxr before SimulationApp.
• Add DummyEmbodiment/DummyObject test doubles plus tests for embodiment
  bbox, graph conversion, and solver placement; add example graph YAMLs
  (kitchen_droid_placement_only, kitchen_embodiment_placement,
  kitchen_pick_and_place).