feat: add direct voxel matrix renderer

[agustin-littlehat]

Summary

• Replace the .vox slice-host renderer with a direct matrix voxel renderer that emits exact visible quad <b> leaves and syncs camera culling.
• Tighten atlas canonical sizing, WebKit stable-triangle fallback, and lossy/rect-cover preprocessing paths.
• Expose boxPolygons across core, React, and Vue, with README/docs and bench investigation tooling updated.

Tests

• pnpm test
• pnpm build