Feature/direct rod solver

[cdinea]

Summary

This PR implements the "Direct Position-Based Solver for Stiff Rods" algorithm from Deul et al. (Computer Graphics Forum / Eurographics 2018) as an extension to the isaaclab_newton package.

Motivation

Standard XPBD solvers using Gauss-Seidel iteration struggle with stiff materials, requiring many iterations to converge. This direct solver exploits the rod's serial (acyclic) topology to solve all constraints simultaneously in O(n) time

Implementation

The solver simulates an inextensible Cosserat rod discretized as a chain of rigid body segments. Key features:

• O(n) direct solver via block-tridiagonal matrix factorization (Thomas algorithm)
• GPU-accelerated Warp kernels for parallel constraint evaluation
• Cosserat rod model with stretch and bend-twist constraints
• Physically meaningful parameters: Young's modulus, density, damping
• Multiple visualization options: Isaac Sim (USD), Matplotlib (standalone)

Files Added

Checklist

• 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 updated the changelog and the corresponding version in the extension's config/extension.toml file
• I have added my name to the CONTRIBUTORS.md or my name already exists there

[greptile-apps]

Skipped: This PR changes more files than the configured file change limit: (1012 files found, 100 file limit)