feat: Level of Detail (LOD) helper in @webgamekit/threejs

[cnotv]

Parent epic

#19

Problem

Distant objects are rendered at full polygon count even when individual triangles are sub-pixel. This wastes GPU vertex throughput. Three.js provides THREE.LOD but there is no shared helper to configure it consistently across scenes.

Goal

A composable LOD factory in packages/threejs/src/ that makes distance-based mesh swapping trivial to adopt, including an "impostor" tier (billboard sprite) for maximum-distance objects.

Plan

1. packages/threejs/src/lod.ts

/**
 * Creates a THREE.LOD object from an ordered array of mesh+distance pairs.
 * @param levels - array of { mesh, distance } sorted near → far
 * @returns THREE.LOD ready to add to the scene
 */
export const createLOD = (levels: LODLevel[]): THREE.LOD => { … }

/**
 * Creates a billboard impostor sprite from a texture for the lowest LOD tier.
 */
export const createImpostorSprite = (texture: THREE.Texture, size: number): THREE.Sprite => { … }

• Validates that levels are sorted ascending by distance
• JSDoc on all exports (required by jsdoc/require-jsdoc rule)

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

3. Apply in one existing scene

• Target: ContinuousWorld or ForestGame — both have distant repeated objects
• Replace furthest-tier objects with impostor sprites
• Verify triangle count drops in the stats panel at distance

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

• createLOD assigns the correct distances to each level
• Levels added in wrong order throw or warn
• createImpostorSprite returns a THREE.Sprite

Acceptance criteria

• createLOD and createImpostorSprite exported from @webgamekit/threejs
• At least one scene uses LOD; triangle count reduces at distance in stats panel
• Unit tests pass
• All exports have JSDoc