feat: GPU instancing helper in @webgamekit/threejs

[cnotv]

Parent epic

#19

Problem

Scenes with many identical meshes (trees, rocks, particles, game tiles) currently create individual THREE.Mesh objects, resulting in one draw call per object. For 1 000+ objects this tanks performance. THREE.InstancedMesh can collapse them to a single draw call but its matrix API is verbose and error-prone.

Goal

A composable helper in packages/threejs/src/ that wraps InstancedMesh with a clean, functional API matching the project's existing patterns.

Plan

1. packages/threejs/src/instancing.ts

/**
 * Creates a managed InstancedMesh with helpers to set/update instance transforms.
 * @param geometry - shared BufferGeometry
 * @param material - shared Material
 * @param transforms - initial array of { position, rotation, scale } objects
 * @returns { mesh, updateInstance, resetInstances }
 */
export const createInstancedMesh = (
  geometry: THREE.BufferGeometry,
  material: THREE.Material,
  transforms: InstanceTransform[]
): InstancedMeshHandle => { … }

• Internally uses a pre-allocated Matrix4 and Euler/Quaternion — no new in update path
• updateInstance(index, transform) mutates the matrix at index and sets instanceMatrix.needsUpdate = true
• resetInstances(transforms) bulk-updates all matrices

2. Export from packages/threejs/src/index.ts

3. Apply in at least one existing scene

• Identify a view that renders many identical objects (e.g. ContinuousWorld tree population in area-population.ts)
• Migrate from individual meshes to createInstancedMesh
• Measure draw call reduction (before/after comment in PR)

4. Tests (packages/threejs/src/instancing.test.ts)

• createInstancedMesh returns a mesh with correct count
• updateInstance sets the matrix correctly at a given index
• Pre-allocated scratch objects are not re-created across calls

Acceptance criteria

• createInstancedMesh exported from @webgamekit/threejs
• No new THREE.* in the update path
• At least one scene migrated; draw calls visibly reduced in stats panel
• Unit tests pass

[cnotv]

Add case to prevent creating multiple unnecesary materials and geometries, more performant materials.
Search or create ESLint rule for ThreeJS for these related cases.