Support Embodiment Placement for Droid/Franka

[qianl-nv]

Summary

Local on-stand USD compose for Franka and Droid to support placement with stand/camera

Detailed description

• Stand and Droid external cameras were detaching on relation reset when spawn and reset layouts differed; parenting them under panda_link0 lets a single write_root_pose_to_sim move the full assembly in Fabric/Kit.
• Franka and Droid now compose cached robot+stand USDs locally (robot_on_stand_utils, embodiment-specific mount helpers) instead of relying on Nucleus combined assets or a separate Droid stand scene entity.
• Remove has_unplaced_auxiliary_prims API since the only use case Droid has been updated to a single USD.
• Added test_robot_on_stand_utils.py and merge test_droid_stand_height.py into it.

Before:

After:

[qianl-nv]

self review 1

[qianl-nv]

self review 2

[greptile-apps]

Greptile Summary

This PR locally composes Franka and Droid robot-and-stand USD assemblies.

• Parents stands and Droid external cameras beneath panda_link0 so root-pose writes move the complete assembly.
• Replaces Droid's separate stand entity and removes the obsolete auxiliary-prim placement guard API.
• Adds composition, stand-height, hierarchy, camera, and root-write tests.

Confidence Score: 3/5

The PR should not merge until composed USD cache paths uniquely represent the requested stand height.

Distinct supported stand-height values can resolve to one rounded filename, allowing a later embodiment construction to overwrite the USD referenced by an earlier configuration and produce order-dependent stand geometry.

Files Needing Attention: isaaclab_arena/embodiments/robot_on_stand_utils.py

Important Files Changed

Filename	Overview
isaaclab_arena/embodiments/robot_on_stand_utils.py	Adds shared USD composition and caching, but rounded cache filenames allow distinct stand-height configurations to overwrite one another.
isaaclab_arena/embodiments/droid/droid.py	Replaces the separate Droid stand with a composed assembly and reparents external cameras under the robot base link.
isaaclab_arena/embodiments/franka/franka.py	Switches both Franka control variants to locally composed robot-on-stand USDs.
isaaclab_arena/environments/relation_solver_interface.py	Removes the auxiliary-prim guard after Droid's stand becomes part of the robot assembly.
isaaclab_arena/tests/test_robot_on_stand_utils.py	Adds simulation tests for composition, stand dimensions, hierarchy, cameras, and root-pose movement.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart LR
  A[Embodiment configuration] --> B[compose_on_stand_usd]
  B --> C[Resolve robot and stand USDs]
  C --> D[Mount stand beneath panda_link0]
  D --> E[Write composed USD to local cache]
  E --> F[ArticulationCfg spawn path]
  F --> G[Root-pose reset moves robot, stand, and cameras]

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_{Reviews (1): Last reviewed commit: "Add shared on-stand USD compose for Droi..." | Re-trigger Greptile}

[qianl-nv]

Addressed all review comments

[zhx06]

LGTM!