十分钟物理：FLIP 流体

Keep particles with position and velocity plus a staggered MAC grid with u and v velocities and solid flags. Each step: integrate gravity on particles, push apart overlapping particles using a spatial hash, handle collisions with walls and a draggable circle obstacle, transfer particle velocities to the grid with bilinear weights, save the grid velocities, run Gauss-Seidel pressure iterations with over-relaxation and a density correction term, then transfer back as a blend of FLIP (add the change) and PIC (copy). Render particles as GL points coloured by local density.