feat(cientos): <Decal> texture atlas + unified material

[alvarosabu]

Follow-up from the <Decal> roadmap (previously tracked in packages/cientos/src/core/abstractions/Decal/ROADMAP.md).

Use case

Today each decal owns its own material, so even after geometry batching draw calls won't collapse — different materials = different draw calls.

Approach

Pack every decal texture into a single atlas (e.g. CanvasTexture) and drive per-decal UV offsets from the atlas mapping. All decals share one material.

• Re-pack on add/remove. Use a free-rect allocator for incremental updates.
• JSON layout schema gains a uvRect per decal.

Friction

• Single set of sampling parameters (magFilter, colorSpace, anisotropy) shared across all decals in the atlas.
• Prerequisite for feat(cientos): <Decal> geometry batching for high decal counts #1412 to actually collapse draw calls.

[linear-code]

TRES-315