Add instances() API for primitive instancing

[aashu2006]

Addresses #8911

Changes

• Add a new instances(count) API for primitive instancing in WebGL.
• Support instancing for primitives including sphere(), box(), plane(), ellipsoid(), cylinder(), cone(), and torus().
• Support instances(count).model(geometry) while preserving existing model(geometry, count) behavior.
• Forward instance counts through the existing rendering pipeline to _drawGeometry().
• Add unit tests covering primitive instancing, model instancing, explicit count precedence, and cleanup of temporary instance state.

PR Checklist

• npm run lint passes
• Inline reference is included / updated
• Unit tests are included / updated