rough-wall-skin-frictionlisted

# Rough Wall Skin Friction (aerodynamics/boundary-layer/rough-wall-skin-friction) Use when the task is estimating the turbulent skin-friction coefficient of a rough flat plate in incompressible flow from the equivalent sand-roughness height: smooth-wall turbulent baseline, friction velocity, roughness Reynolds number k+, surface regime classification, the fully-rough-cf correlation and a non-iterative coefficient selection with a documented log-linear blend, plus the trip-criterion test for a roughness element or trip strip. This leaf implements the classical roughness band model in pure Python, stdlib only, with the module constants from the leaf spec (SMOOTH_K_PLUS = 5.0, FULLY_ROUGH_K_PLUS = 70.0, ROUGH_MIN_X_OVER_KS = 100.0, TRIP_RE_K = 600.0). It pairs with aerodynamics/boundary-layer/boundary-layer-theory, whose smooth-plate correlations supply the smooth-wall baseline anchor, and with aerodynamics/drag-polars/parasite-drag, whose buildup can consume the rough coefficient as the component Cf. ## Domain quick reference - Smooth-wall turbulent baseline (1/7 power law): cf_smooth = 0.0592 / re_x**0.2, with re_x = rho * u_inf * x / mu the local Reynolds number at the fetch station x. The rough surface can only raise friction above this baseline. - Friction velocity: u_tau = u_inf * sqrt(cf / 2), the shear velocity that scales the roughness. - Roughness Reynolds number: k+ = rho * u_tau * k_s / mu, formed on the equivalent sand-roughness height k_s. - Regime ban