fix(newton): split flat-builder site injection per environment

[pascal-roth]

Description

_cl_inject_sites_fallback — the non-replicated/flat-builder path in isaaclab_newton's
NewtonManager, used whenever a scene is built with replicate_physics=False — packed every
environment's matched site index into a single one-element outer list instead of one sublist per
environment. Any consumer indexing per_world[env_index] for env_index > 0 (e.g.
NewtonRaycastSensor._resolve_site_indices) hit an IndexError as soon as a scene had more than
one environment with replicate_physics=False.

The fix groups matches by their env_<N> path segment instead, so the site map has one sublist
per environment, matching the convention already used by the replicated builder path.

Fixes # (issue)

Test plan: Reproduced against a custom multi-environment task with replicate_physics=False
and a robot-attached ray-caster sensor; confirmed the IndexError before the fix and clean
initialization after. Not covered by an automated CI test yet.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Release backport

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

Screenshots

N/A — backend indexing fix, no visual output.

Checklist

Docker and GPU tests run on demand. Push the commits you want tested, then
comment run-ci on the pull request.

• I have read and understood the contribution guidelines
• I have run the pre-commit checks with ./isaaclab.sh --format
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I have added a changelog fragment under source/<pkg>/changelog.d/ for every touched package (do not edit CHANGELOG.rst or bump extension.toml — CI handles that)
• I have added my name to the CONTRIBUTORS.md or my name already exists there

[greptile-apps]

Confidence Score: 4/5

The PR is not safe to merge until the fallback retains support for single-world flat body labels without an environment namespace.

Findings

1.  Flat labels now fail ▶

Summary

• Aligns multi-environment fallback maps with consumers that index sites by environment.
• Introduces a regression for supported flat, single-world body labels that lack an environment namespace.

Diagram

%%{init: {'theme': 'neutral'}}%%
flowchart TD
  A[Match flat-builder body labels] --> B{Label contains env_N?}
  B -->|Yes| C[Group site under environment N]
  C --> D[Build one sublist per environment]
  B -->|No| E[Current change raises ValueError]
  E --> F[Single-world flat-label initialization fails]

Loading

[greptile-apps]

Flat labels now fail

The fallback API supports flat body labels such as Robot/base, and the existing single-body and wildcard tests use labels without an env_<N> segment. This new check raises for those supported single-world scenes, so body-attached sensors fail during site injection instead of receiving a one-world site map. Please preserve the existing one-world behavior when matched labels have no environment namespace.

[isaaclab-review-bot]

Isaac Lab Review Bot

The flat-builder site map is correctly split into per-environment sublists, restoring the structure expected by environment-indexing consumers. The user-visible bug fix is missing the required changelog fragment for isaaclab_newton.

• Design and architecture: Normalizing the non-replicated builder output to the same per-environment structure used by the replicated path is appropriate, while global and per_world site handling remains unchanged.
• API: The corrected local-site map shape matches the existing consumer contract. No supported public API compatibility issue is established by the patch.
• Implementation: The grouping logic directly addresses the multi-environment indexing failure. Before merge, add the repository-required bug-fix changelog fragment for the changed isaaclab_newton source package, or an explicit .skip fragment if appropriate.

Minor fixes needed. Posted 1 actionable finding inline.

Automated review; human maintainers own approval decisions.

[isaaclab-review-bot]

🟡 Warning · Implementation — Missing changelog fragment for isaaclab_newton

Repository rules require one changelog fragment per changed source package for user-visible changes, and this fixes a user-facing IndexError during scene initialization with replicate_physics=False and more than one environment. Only the source file is present in this PR. Add a bug-fix fragment for isaaclab_newton describing the corrected per-environment flat-builder site mapping (or a .skip fragment if intentionally unreleased).