batchIds

Per-vertex _BATCHID from b3dm — one uint16 per vertex (the glTF spec allows UNSIGNED_BYTE / UNSIGNED_SHORT / FLOAT; we normalise to a short, half the size of the original FLOAT path). Bound to the shader as GL_UNSIGNED_SHORT with normalized = false, so the vertex shader sees the actual integer batchId as a float. Null when the source has no _BATCHID attribute. Tiles with more than 65535 batches would overflow — those datasets need a wider format and are out of scope for now.