Add delayed implicit actuator support - #8992

Description

Adds DelayedImplicitActuator and DelayedImplicitActuatorCfg to model command latency while retaining solver-side implicit PD control, as requested by Hexagon. Position, velocity, and feed-forward effort targets share one delay per environment, sampled uniformly from [min_delay, max_delay] on reset in physics steps. Joint-state feedback and solver gains remain current.

The implementation shares the existing delayed PD command-history lifecycle, preserves partial-reset history isolation, and documents startup behavior and configuration. Combining delayed implicit groups with native explicit actuator execution in the same articulation raises an actionable error; use use_newton_actuators=False for that combination. Separate articulations can mix both execution paths: Newton retains the environment-owned physics loop whenever a Lab actuator model needs per-step updates, preventing delays from growing with policy decimation.

Type of change

• New feature (non-breaking change which adds functionality)
• Documentation update

Validation

Tests reuse existing coverage: the native collection test owns the mixed-group rejection, the existing delayed-PD equivalence test owns explicit-delay compatibility, and the mixed-scene timing regression uses the already-live Newton scene. No additional simulation scene builds are introduced. Independent zero/fixed delay, partial reset, per-environment sampling/channel sharing, and invalid configuration boundaries remain covered.

• All 26 collection tests passed with kitless selection (pytest ... -m kitless).
• All 38 Newton actuator integration tests passed, preserving the existing native-only CUDA-graph decimation coverage.
• Four temporary mutation checks proved the retained tests detect lost explicit delay, skipped physics-step updates, cleared Lab registration, and removal of the mixed-group guard. Every mutation was restored afterward.
• Full repository formatting, lint, changelog, and Git LFS pointer checks passed; no hooks skipped.
• Clean Sphinx HTML documentation build passed with warnings treated as errors.

Checklist

• I have read and understood the contribution guidelines
• I have run the pre-commit checks with uv run isaaclab --format
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have validated the changed behavior with focused coverage and applied the test-audit criteria to test changes
• I have added a changelog fragment under source/isaaclab/changelog.d/ and source/isaaclab_newton/changelog.d/, with a minor-version marker for the new actuator API
• My name already exists in CONTRIBUTORS.md

The delayed explicit actuator regression exercises the existing Lab-execution deprecation warning. Existing Newton adapter tests also emit PyTorch export/deprecation warnings; the documentation build was warning-free.