xeokit WebGPU Renderer

Experimental WebGPU renderer backend for xeokit Viewers.

This module exposes the public renderer class and event/parameter contracts so integrations can type against WebGPU without depending on WebGL-specific APIs. The current backend can attach to a Viewer with an initialized WebGPU device, configure View canvases, clear dirty Views, and draw a first narrow depth-buffered path for indexed triangle-family meshes with flat per-mesh color. Materials, edges, points, lines, picking, and snapshots are intentionally not implemented yet.