Fix side-wall particle climbing in SPH collision response

[Testtesttestestes]

Motivation

• Prevent particles from unnaturally climbing up the flask side-walls by reducing tangential cohesive drift applied at near-vertical wall contacts.

Description

• Modify src/sph/update.frag to compute sideWallFactor = smoothstep(0.55, 0.9, abs(normalLocal.x)) and attenuate cohesiveSlip by multiplying it with (1.0 - sideWallFactor) so cohesive tangential drift is reduced on vertical walls.

Testing

• npm run lint completed successfully.
• npm run build completed successfully.
• An automated Playwright screenshot attempt crashed (Chromium SIGSEGV) so no visual artifact was produced.

---

Codex Task