feat: migrate omni.isaac.* imports to isaacsim.* via compat layer

- Add core/compat.py: compatibility module with try/except imports
  supporting both IS 4.x (omni.isaac.*) and IS 5.x+ (isaacsim.*)
- Migrate 152 imports across 47 files from direct omni.isaac.* to core.compat
- Handle class renames: RigidPrim→SingleRigidPrim, GeometryPrim→SingleGeometryPrim,
  XFormPrim→SingleXFormPrim, Articulation→SingleArticulation (aliased for compatibility)
- Add migerate/migrate_imports.py: automated migration script for future use
- Leave debug_draw and env_loader try/except imports as-is

This eliminates ~100 deprecation warnings from our code on IS 5.0,
and future-proofs for IS 6.x when old APIs may be removed.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

workflows/simbox/tools/rigid_obj/asset_usd_converter.py

@@ -83,7 +83,7 @@ if __name__ == "__main__":
     kit = SimulationApp()
 
     import omni
-    from omni.isaac.core.utils.extensions import enable_extension
+    from core.compat import enable_extension
 
     enable_extension("omni.kit.asset_converter")