Adds a ProceduralSky ESM script and a graphics example showcasing a full day/night cycle with dynamic image-based lighting. The daytime atmosphere is a port of the three.js Sky shader (Preetham analytic daylight); this is expanded with an original procedural night-sky model (deep gradient, warm twilight glow, stars, and a moon disk) blended in by time of day, plus a moon key light. Addresses #4378.

Changes:

• scripts/esm/sky/procedural-sky.mjs - reusable ESM script. Renders the visible sky via a skyboxPS chunk override (no custom mesh) and bakes the lighting into the scene env-atlas (RGBM, so no float-renderable dependency). Drives a linked directional light's direction/color/intensity across the day and crossfades to a configurable moon key light at night. The night sky (gradient, twilight glow, stars, moon disk) is an original addition on top of the ported daytime model.
• Graphics example procedural-sky (.example.mjs + .controls.jsx) - a laboratory in a desert with SSAO, HDR bloom, PCSS soft shadows, torch lights at night, and editable time-of-day curves.
• New asset dry-sand-terrain.glb (+ attribution) and example thumbnails.

Examples:

• New: Graphics / Procedural Sky

Credits:

• Daytime atmosphere ported from three.js Sky (MIT) - Simon Wallner, Martin Upitis, Joshua Koo / zz85; underlying model by Preetham, Shirley, Smits (1999).
• "FREE - Dry Sand Terrain" by josevega (CC BY 4.0); "Laboratory" by Sketchfab (CC BY 4.0).

Depends on: #8942 (PCSS hardware-bias fix) for correct WebGPU PCSS shadows.