Add triaxial compression benchmark (Mohr-Coulomb validation)

[SueHeir]

Summary

• New bench_triaxial example validating DEM triaxial compression against Mohr-Coulomb failure theory
• Runs at 4 confining pressures (10, 50, 100, 200 kPa) with servo-controlled lateral walls and constant-velocity axial compression
• Extracts peak stress ratio → friction angle φ and verifies consistency with inter-particle friction (μ = 0.5 → φ ≈ 20–30°)

Files Added

File	Description
examples/bench_triaxial/main.rs	3-stage simulation (insert → relax → compress) with custom wall compression and stress output systems
examples/bench_triaxial/config_{10,50,100,200}kPa.toml	Configs with different servo target forces for each confining pressure
examples/bench_triaxial/validate.py	5 quantitative checks: valid data, stress ratio range, friction angle range, cross-pressure consistency, MC linearity
examples/bench_triaxial/plot.py	Generates stress–strain curves, Mohr circles with failure envelope, q–p diagram
examples/bench_triaxial/run_benchmark.sh	Runs all 4 pressures sequentially
examples/bench_triaxial/README.md	Physics background, setup parameters, simplifications, references

Physics

• Mohr-Coulomb criterion: sin(φ) = (σ₁ − σ₃) / (σ₁ + σ₃)
• Stress computed from wall force accumulators (σ₁ from top wall, σ₃ from lateral servos)
• Gravity zeroed during compression for uniform stress distribution
• Quasi-static loading (inertial number I ≪ 0.01)

Test plan

• cargo build --no-default-features --example bench_triaxial compiles
• cargo test --no-default-features --workspace passes
• No clippy warnings in bench_triaxial code
• Run bash examples/bench_triaxial/run_benchmark.sh and verify output
• Run python3 examples/bench_triaxial/validate.py — all checks pass
• Run python3 examples/bench_triaxial/plot.py — plots generated

🤖 Generated with Claude Code