Add paired ellipsoid nonlinear heave dynamics

[cmccomb]

What changed

• Add a focused instantaneous nonlinear-hydrodynamics mode for one pure-heave body under regular waves, configured through the public wecsim.WEC interface.
• Integrate STL mesh buoyancy, nonlinear Froude–Krylov correction, and quadratic drag with the existing BEM excitation/radiation and PTO dynamics.
• Capture the pinned MATLAB Nonlinear_Hydro/ode4/Regular baseline and gate mesh forces, heave motion, and PTO outputs on all 3,001 samples.

Paired evidence

On the saved MATLAB states, equilibrium mass differs by less than 2e-9 kg, buoyancy by 6e-9 N, drag by 7e-11 N, and heave excitation by 1e-6 N. The independent 150 s Python run differs by at most 5.16 mm heave, 5.86 mm/s velocity, 7.03 kN PTO force, and 7.21 kW absorbed power. MATLAB splits added mass into Simscape mass and an applied force; Python uses the combined BEM effective mass. The new mode rejects unsupported motions and sea states.

Verification

• Paired nonlinear-hydro tests: 2 passed against pinned R2025b output.
• Local suite: 90 passed, 93 skipped (external MATLAB artifacts unavailable in ordinary runs).
• actionlint and git diff --check passed.

SungJun Won's original commits remain in the branch ancestry.