Backport ControllerDifferentialIK and assemble 1.6 changelog

[jcarius-nv]

Description

Backport #4129 to release-1.6, adding the model-based and model-free
ControllerDifferentialIK APIs and their registered example.

The first commit is a clean git cherry-pick -x of main commit
55c824a12f878f010eb5c4b95e2335c6d84f75eb.

The second commit assembles the pending Towncrier fragments into the rolling
1.6.0 section dated 2026-09-04 and removes the consumed fragment and .skip
files. Previously released changelog sections are unchanged.

Please merge this PR without squashing so the source backport and changelog
management commit remain independently traceable. The changelog commit will
later be synchronized to main after the release.

Checklist

• New or existing tests cover these changes
• The documentation is up to date with these changes
• For user-facing changes, a fragment has been added by following the
  changelog fragment instructions

Test plan

uv lock --check
uv run docs/generate_api.py
uv run --extra dev -m newton.tests -k test_controllers_differential_ik -k test_controllers_one_sided_jacobi_svd_solver -k test_examples.example_controller_differential_ik
uvx --from towncrier==25.8.0 towncrier build --draft --version 1.6.0 --date 2026-09-04
uvx --python 3.12 pre-commit run -a

The focused controller run passed 145 CPU/CUDA tests. Changelog validation
confirmed 139 entries in the five supported categories, no duplicate
top-level entries, an empty [Unreleased] section, no remaining fragments,
and no changes to the 1.5.1 or older release history.

New feature / API change

from newton.controllers import ControllerDifferentialIK, DifferentialIKMethod

controller = ControllerDifferentialIK(
    model,
    articulations="panda_*",
    joints=[f"panda_joint{i}" for i in range(1, 8)],
    tool_sites="fr3_hand_tcp",
    bandwidth=5.0,
    ik_method=DifferentialIKMethod.ADAPTIVE_DAMPING,
    adaptive_damping_min=0.02,
    adaptive_damping_max=0.5,
    adaptive_damping_threshold=0.2,
)

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[github-actions]

API review

Detected 59 interface change(s): 59 added, 0 removed, 0 modified.

• Added: newton.controllers.ControllerDifferentialIK (class)
• Added: newton.controllers.ControllerDifferentialIK.Inputs (class)
• Added: newton.controllers.ControllerDifferentialIK.Inputs.bandwidth (constant)
• Added: newton.controllers.ControllerDifferentialIK.Inputs.damping (constant)
• Added: newton.controllers.ControllerDifferentialIK.Inputs.desired_tool_pose_world (constant)
• Added: newton.controllers.ControllerDifferentialIK.Inputs.joint_q (constant)
• Added: newton.controllers.ControllerDifferentialIK.Inputs.joint_qd (constant)
• Added: newton.controllers.ControllerDifferentialIK.Inputs.null_space_damping (constant)
• Added: newton.controllers.ControllerDifferentialIK.Inputs.null_space_stiffness (constant)
• Added: newton.controllers.ControllerDifferentialIK.Inputs.q_des_null (constant)
• Added: newton.controllers.ControllerDifferentialIK.Outputs (class)
• Added: newton.controllers.ControllerDifferentialIK.Outputs.joint_q_target (constant)
• Added: newton.controllers.ControllerDifferentialIK.Outputs.joint_qd_target (constant)
• Added: newton.controllers.ControllerDifferentialIK.controlled_robot_count (property)
• Added: newton.controllers.ControllerDifferentialIK.device (property)
• Added: newton.controllers.ControllerDifferentialIK.input (method)
• Added: newton.controllers.ControllerDifferentialIK.is_graphable (method)
• Added: newton.controllers.ControllerDifferentialIK.max_controlled_dofs (property)
• Added: newton.controllers.ControllerDifferentialIK.model_robot_count (property)
• Added: newton.controllers.ControllerDifferentialIK.output (method)
• … and 39 more change(s).

This check is advisory: the label means API review needed, not that a breaking change is proven.

[codecov]

Codecov Report

❌ Patch coverage is 95.72193% with 32 lines in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...src/controllers/impl/differential_ik/model_free.py	96.91%	11 Missing ⚠️
...rc/controllers/impl/differential_ik/model_based.py	95.00%	9 Missing ⚠️
newton/_src/controllers/joint_selection.py	86.95%	9 Missing ⚠️
newton/_src/controllers/tool_selection.py	95.58%	3 Missing ⚠️

📢 Thoughts on this report? Let us know!