feat(preseed): AAA SPH water — 16384 particles, GPU-built pool mesh, custom water shader
Rework the p_sph DELUGE part for high graphical fidelity:
- 4x particle count (3968 -> 16384); particle record widened to 16 floats to
carry a color-field gradient + persistent foam
- Pool mesh now built and animated entirely on the GPU: a one-shot mesh-init
compute pass writes the 16384 ico-shell templates + index topology into
compute buffers, and a per-frame renderer.mesh.update refreshes the drawn
slab from the scatter output (no per-particle entities, no CPU readback)
- New sph_water surface shader: deep->shallow absorption gradient, animated
micro-ripple normals, fresnel sky reflection, sharp gloss, noise-broken foam
driven by per-vertex foam/surfaceness/speed from the sim
- Force pass computes a neighbor-smoothed surface normal + foam (crest,
aeration, stirrer splash); scatter squashes free-surface shells into a
continuous sheet and enlarges/overlaps shells so the body reads as fluid