Improve default ground plane rendering

[maxkra15]

Description

This draft makes the proposed ground-plane appearance self-contained so it can be reviewed without changing the shared Nucleus asset:

• bundles the review USD, 512 x 512 albedo map, and 512 x 512 roughness map;
• uses 1 m off-white/dark-grey checker cells with NVIDIA-green landmarks every 2 m;
• keeps the base texture's physical scale when GroundPlaneCfg.size changes;
• hides Kit's overlapping editor grid and camera-axis guide by default; and
• preserves uniform scalar roughness/metallic values on textured meshes in Newton RTX with the pinned Newton 1.5.1 release.

No Nucleus or public S3 asset was modified.

Review assets

• Albedo texture
• Roughness texture
• Layered USD

Newton limitation and proposed follow-up

Newton 1.5.1 imports one albedo texture and scalar material values, but its USD viewer drops the per-instance roughness/metallic array for textured meshes. The 49-line Isaac Lab compatibility override preserves uniform values for the pinned release. Newton main now contains generalized scalar material handling, so this override can be removed after the dependency update.

Newton still does not import a connected roughness map: its USD importer explicitly falls back to a scalar, and its Mesh representation exposes one color texture plus scalar roughness. Consequently, the preview USD represents glossy checks as a second visual-only mesh 0.1 mm above the rough base.

That geometry workaround is intentionally presented for review rather than as the desired final architecture. It adds 5,000 quads/10,000 triangles, and resizing the 100 m plane also scales those authored checker cells. The clean final solution is a single UV-scaled quad once Newton supports roughness textures end to end.

Asset publication proposal

If the appearance is accepted, upload it first under an isolated, versioned path such as:

Isaac/IsaacLab/Environments/Grid/default_ground_plane_checker_v1/default_ground_plane.usda

Keeping the USD and its Materials/Textures directory together avoids modifying the existing default_ground_plane.usda or its texture files. The PR can then switch from the bundled review fixture to that path before merge; the normal Isaac-dev-to-public-S3 sync can publish it without an in-place asset mutation.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Release backport

• Backport this pull request to the active release branch after it merges into develop

Screenshots

The robot reflection is visible on the dark checks while the off-white checks remain rough.

Validation

• uv run --frozen python -m pytest source/isaaclab/test/sim/test_default_ground_plane.py -q — 3 passed
• uv run --frozen python -m pytest source/isaaclab/test/sim/test_spawn_from_files.py -k test_spawn_ground_plane -q — 1 passed
• uv run --frozen python -m pytest source/isaaclab/test/terrains/test_terrain_importer.py -k test_plane -q — 4 passed
• uv run --frozen python -m pytest source/isaaclab_visualizers/test/test_newton_adapter.py -q — 56 passed
• uv run --frozen isaaclab -f — passed
• uv build source/isaaclab --wheel — passed; all three ground-plane assets are present in the wheel

Checklist

• I have read and understood the contribution guidelines
• I have run the pre-commit checks
• I have updated relevant docstrings and changelog fragments
• My changes generate no new warnings
• I have added tests that prove the fix is effective
• I have added a changelog fragment for every touched source package
• My name already exists in CONTRIBUTORS.md