Add procedural weather particle system example using Gaussian splats

[mvaligursky]

Procedural infinite weather particle system rendered as Gaussian splats. Particles follow the camera using a deterministic 3D grid with hash-based positioning and animation.

Changes:

• Add GsplatWeather script (scripts/esm/gsplat/gsplat-weather.mjs) — a reusable procedural particle system built on the GSplat format API
• Add weather example with interactive controls under gaussian-splatting category
• Snow and Rain presets with full UI customization (speed, drift, angle, opacity, color, particle size, elongation, density, extents)
• Entity rotation used for wind angle — tilts the entire particle volume so elongated particles (rain) naturally align with the fall direction

API Changes:

• New GsplatWeather script class with properties: extents (Vec3, world-space half-size), density, speed, drift, opacity, color, particleMinSize, particleMaxSize, elongate, followEntity
• rebuild() method to re-create the particle grid after changing extents or density

Examples:

• New gaussian-splatting/weather example with preset dropdown (Snow/Rain), particle property controls, grid configuration, and live particle count stats