feat(cfd): mean viscous dissipation rate

[nochallenge]

What

Adds FlowSolution::mean_dissipation_rate(kinematic_viscosity) — the rate at which the flow irreversibly converts kinetic energy to heat by viscosity:

// eps = nu * <|S|^2> = nu * (rms_strain_rate)^2   (m^2/s^3, per unit mass)

Why

The quantity that closes the turbulent energy budget (the Kolmogorov cascade rate) — the CFD lane's first genuine energy diagnostic. Since |S|^2 = 2 S_ij S_ij, eps = 2*nu*<S_ij S_ij>. It is the strain-squared companion to rms_strain_rate (#428), threading it directly. A pure rotation strains nothing and dissipates nothing; only deformation dissipates.

Test

mean_dissipation_rate_is_viscosity_times_strain_squared (5x5 unit grid):

• (a) worked — pure shear u(y)=gamma*y, |S|=gamma, nu=0.1 -> eps=nu*gamma^2=0.4;
• (b) threads rms_vorticity (feat(cad): Sneed-Folk maximum-projection sphericity #406) (non-tautological) — for a pure shear |S|=|omega|, so eps = nu*rms_vorticity^2;
• (c) solid-body rotation -> no strain -> no dissipation (even though it spins);
• (d) proportional to nu, and zero at nu=0.

Pure method on FlowSolution, no lib.rs change. valenx-cfd-native 129 lib tests (was 128); cargo clippy -p valenx-cfd-native --all-targets -- -D warnings clean. Research-grade turbulence energy-budget diagnostic.