Fix MJCF joint springdamper import

[han-xudong]

Description

Preserve MJCF joint springdamper parameters and forward them to MjSpec, so MuJoCo can derive passive stiffness and damping from effective joint inertia during compilation. The compiled values are also retained when Newton synchronizes mutable joint properties to MJWarp.

Closes #3735

Checklist

• New or existing tests cover these changes
• The documentation is up to date with these changes
• CHANGELOG.md has been updated (if user-facing change)

Test plan

uv run --extra dev -m newton.tests -k test_joint_springdamper_matches_native_mujoco
uvx pre-commit run -a

Bug fix

Steps to reproduce:

1. Import an MJCF hinge with springdamper="0.2 1.5" and a massive child body.
2. Construct SolverMuJoCo and inspect the compiled joint stiffness and DOF damping.
3. Observe that both values were zero instead of the effective-inertia-dependent values produced by native MuJoCo.

Minimal reproduction:

See #3735.

Summary by CodeRabbit

• Bug Fixes

  • Preserved MJCF joint springdamper settings when importing models.
  • Prevented imported spring-damper values from being overridden by default damping or stiffness synchronization.
  • Applied spring-damper settings consistently to ball, linear, and angular joints.

• Tests

  • Added coverage verifying imported spring-damper values match native MuJoCo behavior.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

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Run ID: 6f09b779-e052-4e4a-a2de-ed70707e6c1d

📥 Commits

Reviewing files that changed from the base of the PR and between 0f97d2a and bad4455.

📒 Files selected for processing (4)

• CHANGELOG.md
• newton/_src/solvers/mujoco/kernels.py
• newton/_src/solvers/mujoco/solver_mujoco.py
• newton/tests/test_import_mjcf.py

👮 Files not reviewed due to content moderation or server errors (4)

• newton/_src/solvers/mujoco/solver_mujoco.py
• newton/_src/solvers/mujoco/kernels.py
• newton/tests/test_import_mjcf.py
• CHANGELOG.md

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📝 Walkthrough

Walkthrough

The MJCF importer now preserves joint springdamper values, forwards them to MuJoCo joint creation, and retains compiled stiffness and damping during runtime updates. A regression test compares Newton and native MuJoCo results.

Changes

MJCF springdamper import

Layer / File(s)	Summary
Import metadata and joint creation newton/_src/solvers/mujoco/solver_mujoco.py	Adds the dof_springdamper attribute, loads MJCF values, and applies nonzero values to ball, linear, and angular joints.
Runtime property synchronization newton/_src/solvers/mujoco/solver_mujoco.py, newton/_src/solvers/mujoco/kernels.py	Passes springdamper data to property kernels. Compiled springdamper values prevent overwriting MuJoCo damping and stiffness.
Regression validation and changelog newton/tests/test_import_mjcf.py, CHANGELOG.md	Compares imported values with native MuJoCo and documents the fix.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Sequence Diagram(s)

sequenceDiagram
  participant ModelBuilder
  participant SolverMuJoCo
  participant MuJoCoMjSpec as MuJoCo MjSpec
  ModelBuilder->>SolverMuJoCo: load dof_springdamper metadata
  SolverMuJoCo->>MuJoCoMjSpec: apply springdamper to joint creation
  MuJoCoMjSpec-->>SolverMuJoCo: compile joint stiffness and damping

Loading

Possibly related PRs

• newton-physics/newton#2304: Extends related joint damping migration with MJCF springdamper handling.
• newton-physics/newton#2508: Shares MuJoCo joint damping and stiffness synchronization changes.
• newton-physics/newton#3642: Shares MuJoCo joint-property handling but addresses dof_ref.

Suggested reviewers: adenzler-nvidia

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly describes the primary change: fixing MJCF joint springdamper import.
Linked Issues check	✅ Passed	The changes preserve springdamper values, forward them to MuJoCo, retain compiled values, and add regression coverage for issue #3735.
Out of Scope Changes check	✅ Passed	All changes support the linked issue by updating import, synchronization, documentation, and regression coverage.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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