[Epic] Sprite atlas support

[stormmuller]

Goal

Add sprite atlas (texture packer) support to the rendering module. src/rendering currently only supports loading single-image textures (create-texture-from-image.ts) plus manually-defined nine-slice regions — there's no support for parsing a packed atlas format and looking up sprite regions by name.

Scope (suggested)

• Support at least one common atlas JSON format (e.g. TexturePacker's JSON hash/array format, or a simple custom format if a standard one adds too much surface area initially).
• A way to reference a sprite by name from an atlas and get back its UV/pixel region, feeding into the existing sprite component/rendering path.
• Should compose with the existing instanced sprite-batching so atlas-packed sprites still batch efficiently (this is usually the actual point of atlases — fewer texture binds per draw call).
• Consider whether this should also support atlas-packed sprite-sheet animations (src/animations), since sprite-sheet frame animation already exists and would benefit from atlas-backed frames.

Why this is a milestone, not a single issue

Touches asset loading (atlas parsing), rendering (region lookup + batching), and potentially animations (atlas-backed animation frames) — worth sequencing as its own set of sub-issues rather than one PR.

Acceptance criteria (top-level)

• An atlas can be loaded and individual sprite regions referenced by name
• Atlas-packed sprites still batch efficiently in the renderer
• Documented in documentation-site/docs/docs/rendering with a demo