Curriculum-Showcase: Genesis NaN constraint after teleport-on-terrain

[KraHsu]

Symptom

`uv run genelab play GeneLab-Curriculum-Showcase-v0 --vis --steps 500` crashes mid-run with:

```
genesis.GenesisException: Invalid constraint forces causing 'nan'. Please decrease Rigid simulation timestep.
```

Full traceback (relevant frames):

```
File ".../genelab_showcase/runner.py", line 100, in play
env.step(action)
File ".../src/genelab/envs/manager_based_rl_env.py", line 267, in step
self._scene.step()
File ".../src/genelab/scene/interactive_scene.py", line 218, in step
self._gs_scene.step()
File ".../genesis/engine/solvers/rigid/rigid_solver.py", line 1030, in check_errno
gs.raise_exception("Invalid constraint forces causing 'nan'. ...")
```

Likely cause

The curriculum runner teleports 8 of 16 G1 envs 6.0 m forward of spawn every 30 control steps to trip the `walked > distance_threshold` branch. The instant teleport drops the robot onto a fresh `RandomRough` sub-terrain at `vis_dt = 0.01` with `substeps=1` (per `curriculum/env_cfg.py`); the rigid solver runs into degenerate constraints on the first post-teleport contact and produces NaN.

Confirmed working

The other five showcase tasks run `exit=0` to completion with the same Genesis build:

Task	Status
Sensors / RayCast / Contact / Terrain / Actuator	✅ pass
Curriculum	❌ NaN constraint mid-run

Suggested investigation

1. Drop `dt` to 0.005 or set `substeps=2` in `examples/genelab_showcase/src/genelab_showcase/curriculum/env_cfg.py`.
2. Soft-teleport (multi-step relocate instead of single-frame jump), or set velocity to zero before the teleport.
3. Check whether `articulation.write_root_state` is being called before or after the contact-pair refresh — a stale contact set at the new pose could explain the NaN.

Repro

```bash
git checkout feat/showcase-and-foundation-docs # or any branch with the showcase package
uv pip install -e examples/genelab_showcase
uv run genelab play GeneLab-Curriculum-Showcase-v0 --vis --steps 500
```

Surfaced while smoke-testing PR #29's human verification checklist.