Reduce raymarch iterations from 28 to 18 and fBm octaves from
4 to 3 (with step size 0.55->0.85 to preserve cloud coverage).
Per-pixel cost drops by ~55% while visual density stays similar.
Raymarches a horizontal cloud layer (28 steps, 4-octave 3D fBm).
Camera path is sinusoidal in altitude so the view dips into the
cloud deck at intervals for a 'fly through the edges' feel.
Density threshold from weather: sunny -> sparse, stormy -> overcast.
Renderer is selected via CONFIG.renderer ('clouds2d' or 'clouds3d').
Richard Zink