Add CFL limit to timestep constraints

[ebchin]

• Extends the Common Plane penalty timestep vote with an optional CFL-like contact-stiffness limit, while keeping the original gap-based and velocity-projection penetration checks
• Test for new CFL limit
• Improves edge case handling: timestep calculations now use explicit zero-velocity guards instead of perturbing velocities, and nonpositive element thickness in element-penalty contact now causes the pair to be skipped before any contact forces are assembled