Share Newton barrier line-search step scaling

[jslee02]

Summary

• Share Newton-barrier line-search step-scale normalization in detail/newton_barrier.
• Route rigid IPC Newton step scaling and deformable/world CCD limiters through the shared helper.
• Keep line-search result payloads, limiting candidate identities, and zero-step diagnostics variant-local.

Motivation / Problem

• PLAN-083 Phase 3 promotes only proven second-use solver contracts.
• Rigid IPC and deformable/world CCD limiters already shared the same finite [0, 1] step-bound normalization shape, with deformable paths also stepping strictly inside the CCD bound before applying a candidate.
• Sharing this narrow policy removes duplicated feasibility plumbing without promoting full projected-Newton status or line-search result semantics.

Changes / Key Changes

• Added newton_barrier::makeLineSearchStepScale() for finite clamping plus optional safety scaling.
• Added newton_barrier::makeInteriorLineSearchStepScale() for the strictly-inside CCD fraction used by deformable/world line searches.
• Replaced duplicate rigid IPC and world-step local calculations with the shared helpers.
• Added focused Newton-barrier primitive tests and updated the active PLAN-083 dev-task handoff evidence.

Testing

• pixi run lint
• pixi run -- cmake --build build/default/cpp/Release --target test_newton_barrier_primitives test_rigid_ipc_barrier test_world --parallel 8
• pixi run -- ctest --test-dir build/default/cpp/Release --output-on-failure -R '^(test_newton_barrier_primitives|test_rigid_ipc_barrier|test_world)$'
• pixi run build
• pixi run test-unit
• pixi run test-all
• pixi run -e cuda test-all

Breaking Changes

• None
• Internal detail-only helper; no public API or migration required.

Related Issues / PRs (backports)

• Follows Share line-search CCD option adapter #2942.
• Part of PLAN-083 / docs/dev_tasks/unified_newton_barrier_multibody.
• Backport: N/A; DART 7 simulation work targeting main.

---

Checklist

• Milestone set (DART 7.0 for main, DART 6.17.1 for release-6.17)
• CHANGELOG.md updated if required (N/A: internal detail helper and dev-task evidence only)
• Add unit tests for new functionality
• Document new methods and classes (N/A: internal detail helper; dev-task evidence updated)
• Add Python bindings (dartpy) if applicable (N/A)

[jslee02]

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Codecov Report

❌ Patch coverage is 96.29630% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 84.24%. Comparing base (481302f) to head (817e904).

Files with missing lines	Patch %	Lines
...t/simulation/detail/newton_barrier/line_search.hpp	93.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2943      +/-   ##
==========================================
+ Coverage   84.23%   84.24%   +0.01%     
==========================================
  Files         684      684              
  Lines       81088    81082       -6     
==========================================
+ Hits        68302    68306       +4     
+ Misses      12786    12776      -10     

Flag	Coverage Δ
unittests	84.24% <96.29%> (+0.01%)	⬆️

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Files with missing lines	Coverage Δ
dart/simulation/compute/world_step_stage.cpp	92.90% <100.00%> (+0.14%)	⬆️
dart/simulation/detail/rigid_ipc_barrier.cpp	94.18% <100.00%> (+0.06%)	⬆️
...t/simulation/detail/newton_barrier/line_search.hpp	96.96% <93.33%> (-3.04%)	⬇️

... and 1 file with indirect coverage changes

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