Fix root fixing on stages without omni.physx

[ooctipus]

Fix root fixing on stages without omni.physx

What

• Adds create_world_fixed_joint(articulation_prim, stage) to
  source/isaaclab/isaaclab/sim/schemas/schemas.py: a pure-USD helper that authors the
  world-attached UsdPhysics.FixedJoint used to fix an articulation root link. It mirrors
  the behavior of omni.physx.scripts.utils.createJoint(joint_type="Fixed", from_prim=None, to_prim=...): unique joint naming, walking up to the first writable ancestor for
  instanceable/prototype/instance-proxy roots, anchoring localPos0/localRot0 at the root
  link's world pose, and setting break force/torque to the same effectively-unbreakable
  MAX_FLOAT value.
• modify_articulation_root_properties (the fix_root_link spawn path) now calls this
  helper instead of importing omni.physx.scripts.utils.
• PhysicsManager.fix_articulation_root delegates to the same helper instead of carrying
  its own inline joint-authoring copy, so there is a single authoring path for every
  backend.
• Adds a kitless regression test,
  source/isaaclab/test/sim/test_schemas_kitless_fixed_joint.py, which runs on an
  in-memory USD stage without launching Isaac Sim / Kit.
• Adds a changelog fragment: source/isaaclab/changelog.d/schemas-kitless-fixed-joint.rst.

Why

Spawning a fixed-base articulation (fix_root_link=True) previously imported
omni.physx.scripts.utils inside modify_articulation_root_properties. On stages where
omni.physx is not available (kitless / usd-core-only setups, e.g. Newton backends), that
import raises ModuleNotFoundError, breaking the spawn path. Authoring the joint directly
with plain USD removes the omni.physx dependency while preserving the authored result.
Sharing the helper with PhysicsManager.fix_articulation_root also removes a duplicated
copy of the joint-authoring logic.

This is a standalone correctness fix plus a regression test; no performance claims.

Evidence (existing upstream workflow)

Measured with the repository's own benchmark, identical command on the PR branch and its base (upstream/develop @ 5c42d39):

./isaaclab.sh -p scripts/benchmarks/runtime.py --task Isaac-Velocity-Flat-AnymalD --num_envs 1024 --num_frames 300 --seed 7

Platform: RTX 5090, Newton backend, CUDA graphs on.

arm	wall (300 iters × 1024 envs)	ms/iter
develop base	3.811 s	12.70
this PR	n/a — CPU-side USD authoring fix, no runtime path touched	—

Runtime evidence is not applicable; the change fixes fix_root_link on kit-less stages. Correctness evidence: the new regression test (test_schemas_kitless_fixed_joint.py, 2 passed) exercises the plain-USD path end-to-end.

🤖 Generated with Claude Code

[ooctipus]

Closing — this is already covered by #6536.