Stabilize voxel face culling

[agustin-littlehat]

Summary

• mount direct voxel leaves under persistent signed-face wrappers so culling swaps face hosts instead of rebuilding every leaf
• remove unused voxel slice planner plumbing and stale voxel report bench script
• keep polycss/react/vue voxel CSS selectors aligned with the current direct renderer path

Verification

• pnpm test && pnpm build
• pnpm bench:build