Implement cloth stretch constraints

[0dm]

Summary

Implement XPBD stretch constraints for cloth edges.

Why

Stretch constraints are the core inextensibility mechanism for cloth. Without them, there is no useful cloth behavior.

Scope

• Generate stretch constraints from cloth edge topology
• Store rest length per constraint
• Solve with XPBD compliance
• Support pinned/static particles correctly
• Add basic tuning parameters for stiffness/compliance

Acceptance Criteria

• A simple hanging cloth patch preserves edge lengths within expected tolerance.
• Pinned vertices remain fixed.
• Constraint behavior is stable across multiple substeps.