Audit RM3 PTO extension convergence at 0.0125 s

[cmccomb]

Summary

Extend the existing RM3 PTO-extension float and spar no-wave free-decay audit from 0.1/0.05/0.025 s to one finer 0.0125 s MATLAB and Python pair. The published inputs, 30 s duration, ode4 solver, initial 5 m PTO stroke, and zero PTO force remain fixed; only simu.dt changes in the diagnostic source run.

Predeclared gates

• MATLAB 0.025-to-0.0125 s maximum active heave/PTO position and speed changes must each be below 75% of the 0.05-to-0.025 s change.
• Independent Python at 0.0125 s must stay below 9 mm and 17 mm/s float-heave error, and below 1.5 mm and 0.4 mm/s spar-heave error. PTO stroke/speed use the same limits.
• Each new Python/source active motion error must be below 75% of its 0.025 s error. Inactive-body heave and zero PTO force/power remain gated.

The current RM3 PTO-extension cases remain qualified_motion; this experiment tests convergence and does not relabel the published 0.1 s cases. Do not relax these gates merely to obtain a pass.

Local checks

• git diff --check
• Python test compiles and reports its expected skip without MATLAB/application assets.