🌌 Cosmic Web to Spiral Disk Lab: Zel'dovich Collapse, NFW Halo, Tidal-Torque Spin and Density Wave

Grow a cosmic web from small ripples with the Zel'dovich approximation and LCDM growth, collapse a node into a halo, then watch a disc galaxy form inside it: the halo's mass and spin set the disc, a dark halo flattens the rotation curve and an imposed spiral density wave sweeps through 20,000 stars. Includes Lindblad resonances, Toomre Q and a perturbation-strength experiment.