resource model guide

The read-first guide for working with meshes, textures, materials, and shaders. Explains the three forms every GPU-backed resource takes (authored asset, CPU-decoded data, live GPU resource), the Luau to CPU to GPU upload path, residency and memory management (load / unload / upload / drop / isResident), what keeps a runtime resource alive and how it is released (renderer.references, renderer.collect, renderer.hold / renderer.release, and the collection a root scene load runs), the asset material vs runtime material distinction (gold.material vs renderer.material.create), the asset graph (material to shader to texture), render textures, and surfaces.

Surfaced through guides { path: "core/resource-model" }. Content lives in guide.md.