experiment: evaluate bounded Rust single-wheel soil numerics

[StefanAleksa]

Adds an isolated, dependency-free Rust numerical experiment for #52, before any production soil-backend decision.

Prescribed single-wheel load/slip sweeps use a simplified circular-foundation pressure model and shear-displacement proxy. Independent closed-form load and uniform-patch shear integrals check the quadrature. The README records assumptions, synthetic inputs, measured results, and a bounded decision: continue Rust research, but do not adopt this model as production physics.

This is NOT an Avian scene, validated lunar regolith, net drawbar-pull model, or complete P0. Production baseline, physical calibration, persistent terrain state, and Modelica/USD/Rhai integration remain open. No production code/dependencies are changed.

Validation:

• Five numerical tests failed against the initial stub, then passed.
• PowerShell reproduction script passed: 20 cases, identical repeated CSV values.
• Grid-refinement and analytic reference checks passed.
• Skill catalogue validation passed (42 skills).
• Staged whitespace/source-only review passed; all binaries/outputs are ignored under target/soil-spike.

Related design: #55. Refs #52; does not close it.