improve: warm-start pressure solver from previous frame

[jayf0x]

Problem

Each frame, the pressure Jacobi solver starts from stale/garbage pressure. With pressureIterations=1 (default), it barely converges. The divergence changes slowly between frames, so the previous converged pressure is a good initial guess.

Proposed change

Remove the explicit pressure-FBO clear before the Jacobi solve loop. Keep the previous frame's pressure as the starting point — warm-starting means even 1 iteration produces a much better result.

Acceptance criteria

• Pressure FBO is NOT cleared before the solve loop
• Jacobi iterations run from previous frame's pressure
• Fluid looks smoother/more divergence-free at pressureIterations=1
• No new FBOs added

Files likely touched: src/core/simulation.ts